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can vary widely in length and diameter, which prevents the plant from being cross-pollinated with a different species (Figure 18.22). Figure 18.22 Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by hummingbirds. When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. This is called hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own; this is called hybrid sterility. Habitat Influence on Speciation Sympatric speciation may also take place in ways other than polyploidy. For example, consider a species of fish that lives in a lake. As the population grows, competition for food also grows. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them. This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 18.23 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources. Figure 18.23 Cichlid fish from Lake Apoyeque, Nicaragua, show evidence |
of sympatric speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that the lake was populated only 100 years ago by a single population of cichlid fish. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and scientists believe these populations may be in an early stage of speciation. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 743 18.3 | Reconnection and Rates of Speciation In this section, you will explore the following questions: • What are the pathways of species evolution in hybrid zones? • What are the two major theories on rates of speciation? Connection for AP® Courses Speciation can both occur gradually over time in small steps or in bursts of change known as punctuated equilibrium. With punctuated equilibrium, a species may remain unchanged for long periods of time. The primary influencing factor on changes in speciation rate is environmental change. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.C Essential Knowledge Life continues to evolve within a changing environment. 1.C.1 Speciation and extinction have occurred throughout Earth’s history. Science Practice 5.1 The student can analyze data to identify patterns or relationships. Learning Objective 1.20 The student is able to analyze data related to questions of speciation and extinction throughout the Earth’s history. Speciation occurs over a span of evolutionary time, so when a new species arises, there is a transition period during which the closely related species continue to interact. Reconnection After speciation, two species may continue interacting indefinitely or even recombine. Individual organisms will mate with any nearby individual who they are capable of breeding with. An area where two closely related species continue to interact and reproduce, forming hybrids, is called a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the reproductive barriers (Figure 18.24). If |
the hybrids are less fit than the parents, reinforcement of speciation occurs, and the species continue to diverge until they can no longer mate and produce viable offspring. If reproductive barriers weaken, fusion occurs and the two species become one. Barriers remain the same if hybrids are fit and reproductive: stability may occur and hybridization continues. 744 Chapter 18 | Evolution and Origin of Species Figure 18.24 After speciation has occurred, the two separate but closely related species may continue to produce offspring in an area called the hybrid zone. Reinforcement, fusion, or stability may result, depending on reproductive barriers and the relative fitness of the hybrids. What are three different pathways that species evolution may take in hybrid zones? a. stability, fusion, reinforcement b. allopatric speciation, sympatric speciation, fusion c. convergent evolution, divergent evolution, no evolution d. natural selection, genetic drift, gene flow Hybrids can be either less fit than the parents, more fit, or about the same. Usually hybrids tend to be less fit; therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process called reinforcement. This term is used because the low success of the hybrids reinforces the original speciation. If the hybrids are as fit or more fit than the parents, the two species may fuse back into one species (Figure 18.25). Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some hybrid individuals; this is classified as stability because no real net change is taking place. Varying Rates of Speciation Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: gradual speciation model and punctuated equilibrium model. In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward (Figure 18.25). This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism. This OpenStax book is available for free at http://cn |
x.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 745 Figure 18.25 In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time. Describe a situation in which punctuated equilibrium is more likely to take place. a. There is a significant change in the environment over time, such as the breakup of a supercontinent due to tectonic activity. b. A species that has a competitor outcompetes it and drives it to extinction, freeing up more resources. c. There is a sudden and significant change in the environment, such as a volcanic eruption that divides a population that once shared a habitat. d. There is a stable and unchanging environment in which a species can flourish. The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form. 746 Chapter 18 | Evolution and Origin of Species Visit this website (http://openstaxcollege.org/l/snails) to continue the speciation story of the snails. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 747 KEY TERMS acquired characteristics modifications caused by an individual’s environment that can be inherited by its offspring adaptation heritable trait or behavior in an organism that aids in its survival and reproduction in its present environment adaptive radiation speciation when one species radiates out to form several other species allopatric speciation speciation that occurs via geographic separation allopolyploid polyploidy formed between two related, but separate species aneuploidy condition of a cell having an extra chromosome or missing a chromosome for its species autopolyploid polyploidy |
formed within a single species behavioral isolation type of reproductive isolation that occurs when a specific behavior or lack of one prevents reproduction from taking place convergent evolution process by which groups of organisms independently evolve to similar forms dispersal allopatric speciation that occurs when a few members of a species move to a new geographical area divergent evolution process by which groups of organisms evolve in diverse directions from a common point gametic barrier prezygotic barrier occurring when closely related individuals of different species mate, but differences in their gamete cells (eggs and sperm) prevent fertilization from taking place gradual speciation model model that shows how species diverge gradually over time in small steps habitat isolation reproductive isolation resulting when populations of a species move or are moved to a new habitat, taking up residence in a place that no longer overlaps with the other populations of the same species homologous structures parallel structures in diverse organisms that have a common ancestor hybrid offspring of two closely related individuals, not of the same species hybrid zone area where two closely related species continue to interact and reproduce, forming hybrids natural selection reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change polyploidy gametes with extra chromosomes postzygotic barrier reproductive isolation mechanism that occurs after zygote formation prezygotic barrier reproductive isolation mechanism that occurs before zygote formation punctuated equilibrium model for rapid speciation that can occur when an event causes a small portion of a population to be cut off from the rest of the population reinforcement continued speciation divergence between two related species due to low fitness of hybrids between them reproductive isolation situation that occurs when a species is reproductively independent from other species; this may be brought about by behavior, location, or reproductive barriers speciation formation of a new species species group of populations that interbreed and produce fertile offspring sympatric speciation speciation that occurs in the same geographic space temporal isolation differences in breeding schedules that can act as a form of prezygotic barrier leading to reproductive 748 isolation Chapter 18 | Evolution and Origin of Species theory of evolution explains how populations change over time and how life diversifies the origin of species variation genetic differences among individuals in a population vestigial structure physical structure present in an organism but that has no apparent function and appears to be from a functional structure in a distant ancestor vicariance allopatric speciation that occurs when something in the environment separates organisms of the same species into separate groups CHAPTER SUMMARY 18.1 Understanding Evolution Evolution is |
the process of adaptation through mutation which allows more desirable characteristics to be passed to the next generation. Over time, organisms evolve more characteristics that are beneficial to their survival. For living organisms to adapt and change to environmental pressures, genetic variation must be present. With genetic variation, individuals have differences in form and function that allow some to survive certain conditions better than others. These organisms pass their favorable traits to their offspring. Eventually, environments change, and what was once a desirable, advantageous trait may become an undesirable trait and organisms may further evolve. Evolution may be convergent with similar traits evolving in multiple species or divergent with diverse traits evolving in multiple species that came from a common ancestor. Evidence of evolution can be observed by means of DNA code and the fossil record, and also by the existence of homologous and vestigial structures. 18.2 Formation of New Species Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). Both pathways isolate a population reproductively in some form. Mechanisms of reproductive isolation act as barriers between closely related species, enabling them to diverge and exist as genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote, whereas postzygotic barriers block reproduction after fertilization occurs. For a new species to develop, something must cause a breach in the reproductive barriers. Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes (polyploidy). Autopolyploidy occurs within a single species, whereas allopolyploidy occurs between closely related species. 18.3 Reconnection and Rates of Speciation Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid zones. Organisms reproduce with other similar organisms. The fitness of these hybrid offspring can affect the evolutionary path of the two species. Scientists propose two models for the rate of speciation: one model illustrates how a species can change slowly over time; the other model demonstrates how change can occur quickly from a parent generation to a new species. Both models continue to follow the patterns of natural selection. REVIEW QUESTIONS 1. Which scientific concept did Charles Darwin and Alfred Wallace independently discover? a. mutation b. natural selection c. overbreeding d. sexual reproduction 2. Which of these statements about a natural principle that points to the inevitability of natural selection is false? a. Most characteristics of organisms |
are inherited. b. Offspring vary among each other in regard to their characteristics. c. Some generations of offspring do not need to compete for resources. d. Certain traits will be better represented in the next generation. 3. Which is the best definition of adaptation? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 749 a. a trait or behavior that aids an organism’s survival and reproduction 9. Which statement best describes the relationship between the theory of evolution and the origin of life? b. a heritable trait or behavior that aids an organism’s survival and reproduction a. The theory includes an explanation of life’s origins. c. a trait or behavior that aids a population’s b. The theory cannot explain the origin of life. survival and reproduction d. a heritable trait or behavior that aids a population’s survival and reproduction c. The theory does not try to explain the origin of life. d. The theory does not contribute understanding to 4. Which is an example of an adaptation? pre-life processes. a. The better nutrition of a human helps her grow taller. 10. Which best describes what happens when an antibiotic is applied to a population of bacteria? b. The webbed feet of a duck help it swim. a. The bacteria develops resistance to the antibiotic c. The urban location of a raccoon helps it find food. d. The large leaves of a desert plant require more water. 5. Which of the processes described is divergent evolution? a. Groups of organisms evolve in different directions from a common point. in direct response to its application. b. The bacteria’s genetic material mutates in response to the antibiotic, resulting in resistance. c. A gene for resistance, already present in the population, decreases in frequency. d. A gene for resistance, already present in the population, increases in frequency. 11. Which is the best definition of species? b. A new species develops rapidly when an event a. A group of individual organisms with significant cuts off a portion of a population. genetic similarities c. Groups of organisms independently evolve to similar forms. d. A species evolves when a few members move to b. a group of individual organisms with significant genetic similarities that share external and internal characteristics a new geographical area. c. a group of individual organisms that interbreed 6. Which situation is most |
likely an example of convergent evolution? d. a group of individual organisms that interbreed and produce viable, fertile offspring a. Some fish that live in total darkness have eyes. b. Hawks and other birds have feathers. c. Worms and snakes both move without legs. d. Flowers that look very different have the same reproductive organs. 7. What are homologous structures? 12. What do scientists focus on to distinguish between species? a. ecological niches b. morphological differences c. reproductive barriers d. genetic changes a. physical structures that have no apparent 13. Which are two primary sources of genetic variation? function b. parallel structures in diverse organisms c. physical structures that are used only occasionally d. similar structures in diverse organisms 8. Which of the following are two examples of vestigial structures? a. gills in fish and parts of the throat in humans b. butterfly wings and dragonfly wings c. hind leg bones in whales and leaves on some cacti d. shark fins and dolphin fins a. mutations and sexual reproduction b. c. isolation and sexual reproduction sexual reproduction and asexual reproduction d. migration and sexual reproduction 14. Which statement best describes the relationship between genetic variation and speciation? 750 Chapter 18 | Evolution and Origin of Species a. Without genetic variation, speciation would occur more slowly. b. Without genetic variation, speciation would not be possible. c. Genetic variation influences sympatric speciation, but not allopatric speciation. d. There is no relationship between genetic variation and any form of speciation. 15. Which statement about postzygotic barriers is false? a. They occur after fertilization. b. They include hybrids that are sterile. c. They include hybrid organisms that don’t survive the embryonic stage. d. They include reproductive organ incompatibility. 16. Which situation is an example of a prezygotic barrier? a. Two species of fish produce sterile offspring. b. Two species of flowers attract different pollinators. c. Two species of insects mate, but the zygote does not survive. d. Two species of lizards mate, but the offspring dies before reproducing. 17. Which situation would most likely lead to allopatric speciation? a. A flood causes the formation of a new lake b. A storm causes several large trees to fall down. c. A mutation causes a new trait to develop. d. An injury causes an organism to seek out a new food source. |
18. What is the main difference between an autopolyploid individual and a allopolyploid individual? a. number of extra chromosomes b. c. functionality of extra chromosomes source of extra chromosomes d. number of mutations in the extra chromosomes a. b. c. It leads to multiple species forming from one parent species. It only occurs on or around island archipelagos. It requires a population to disperse from its parent species. d. It is a special kind of sympatric speciation. 20. Which is least likely to be a factor that increases the probability of speciation by adaptive radiation? a. There are vacant ecological niches nearby. b. Genetic drift in a population increases. c. There are isolated regions with suitable habitats. d. There are few competitor species. 21. In a hybrid zone, in addition to interacting, what else do two closely related species do? a. compete b. c. d. reproduce transition fuse 22. Which situation means reinforcement is more likely to occur in the hybrid zone? a. The hybrid offspring are more fit than the parent species. b. Reproductive barriers weaken. c. The hybrid offspring are about as fit as the parent species. d. Reproductive barriers strengthen. 23. Which of the following statements is false? a. Graudal speciation and punctuated equilibrium both result in the divergence of species. b. Punctuated equilibrium is most likely to occur in a large population in a stable environment. c. d. In the punctuated equilibrium model, gradualism is not excluded. In the gradual speciation model, traits change incrementally. 19. What is unique about speciation due to adaptive radiation? 24. Which component of speciation would be least likely to be a part of punctuated equilibrium? a. a division in populations b. a change in environmental conditions c. ongoing gene flow d. a number of mutations occuring at once CRITICAL THINKING QUESTIONS 25. What conclusions can you draw about the relationship between the way in which the present-day theory of evolution developed and the credibility of the theory? Explain your thinking. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 751 a. When the theory of evolution was first proposed, it met with a lot of criticism and disbelief, but it is widely supported today. Theories that have withstood a larger amount of criticism are more credible than |
those that are accepted easily b. The theory of evolution has its foundation in both biological and geological observations, making it a more credible theory because it can explain more about the world c. The theory of evolution relies on the heritability of traits, but the mechanism of this inheritance was not understood when the theory was developed. This reduces the credibility of the theory because the people who created it did not understand how it worked d. It is meaningful that two naturalists working independently from each other offered the same explanation for the same set of phenomena. When two people independently look at the same evidence and come to the same conclusion, this reinforces the credibility of that conclusion 26. Describe how an adaptation, such as better running speed, relates to natural selection. a. Natural selection produces beneficial adaptations, such as better running speed, in individuals that run more frequently b. Natural selection randomly mutates individuals’ genetic code until it produces beneficial adaptations, such as better running speed c. Natural selection produces adaptations, such as better running speed, to help individuals survive and reproduce d. Natural selection reproduces individuals with favorable genetic traits-such as the adaptation of better running speed-over time. 27. Give an example of convergent evolution and explain how it supports the theory of evolution by natural selection. a. An example of convergent evolution is the development of the same function, swimming, in organisms that live in different parts of the globe, such as Arctic beluga whales and Antarctic right whales. The fact that organisms that do not come in contact with each other have developed the same traits suggests that natural selection can produce similar adaptations in organisms who share a similar environment b. An example of convergent evolution is the set of adaptations, such as better running speed or more efficient hunting, developed by a species in response to competition with a new species that moves into the same region. The fact that a species adapts after it comes into contact with a competitor suggests that natural selection works more quickly with higher selective pressures. c. An example of convergent evolution is the development of an ancestral structure, a limb, into two different modern structures, such as a hand and a flipper. The fact that natural selection can cause a structure to develop down two different pathways due to different environmental conditions supports the theory of evolution d. An example of convergent evolution is the development of the same function, flying, in organisms that do not share a recent common ancestry, such as insects and birds. The fact that wings that allow flight have developed from very different original structures suggests that |
the process of natural selection can produce similar adaptations in two very different types of organisms who share a similar environment 28. Why do scientists consider vestigial structures evidence for evolution? a. Vestigial structures are the result of convergent evolution, so they are good evidence that natural selection act similarly in similar environmental conditions. b. Vestigial structures are the result of common ancestry, so they are good evidence that different populations of organisms evolved from a common point. c. Vestigial structures are the result of convergent evolution, so they are good evidence for an end goal to evolution. d. Vestigial structures are the result of common ancestry, so they are good evidence for a common origin of all life. 29. Reproduction in sexually-reproducing organisms occurs when two sex cells, or gametes, fuse. In fish, this occurs when sperm swim through the water to find the ovum. In flowers, pollen is dispersed through the air and carried to another flower. Explain what evolutionary 752 Chapter 18 | Evolution and Origin of Species adaptations for reproduction occur in humans, based on the fact that we are land-based animals. a. Genetic variation would increase and speciation would be possible b. Genetic variation would increase and speciation would not be possible. c. Genetic variation would decrease and speciation would be possible. d. Genetic variation would decrease and speciation would not be possible. 35. What role do prezygotic and postzygotic barriers play in speciation? a. Prezygotic and postzygotic barriers allow for the formation of less-fit hybrids that reinforces speciation. b. Prezygotic and postzygotic barriers prevent interbreeding of species such that there is no gene flow between them. c. Prezygotic and postzygotic barriers prevent migration of the two species, causing them to remain in contact with each other and begin to interbreed. d. Prezygotic and postzygotic barriers are present only in newly-formed species, allowing scientists to identify the time of divergence of the species. 36. A population of flowers was separated into two subpopulations when a new river cut through the plain in which they were growing. The number of interbreeding events per year for the two subpopulations of flowers is shown in the graph below. Twenty-four years after they were separated, can you conclude that the two subpopulations of flowers have become new species? Why or why not? 30. While examining |
the human genome, you find a gene that is not homologous to any other organisms known to man. You conclude that this gene must be unique to the human species and could not have evolved from another organism. Would this discovery suggest that humans do not share a common ancestor with all other organisms on Earth? Explain your answer. 31. Mutations in the glucose 6-phosphate dehydrogenase (G6PD) gene can cause a rare anemia when inherited. However, homozygotes with this mutation are less prone to malaria infection, a disease that historically was the most widespread deadly disease among humans. Predict how this mutation would affect the fitness of individuals living in countries where malaria is endemic. 32. How does the scientific meaning of “theory” differ from the common vernacular meaning? a. A scientific theory is a hypothesis that needs to be tested, whereas people often use theory to mean a simple guess. b. A scientific theory is a statement that has been proven correct, while people often use it to mean a statement that has not yet been verified. c. A scientific theory is a thoroughly tested set of explanations for a body of observations of nature, while people often use it to mean a guess or speculation. d. A scientific theory is a random guess, while people often use it to mean a statement that is somewhat based in fact. 33. Why is having a way of defining species and distinguishing between them important for the study of evolution? a. A distinction between species allows scientists to understand the common origin of all species. b. A common definition of species allows scientists to agree on all aspects of the theory of evolution. c. Divergence can only occur at the species level: it does not occur to larger taxa. Therefore it is important to know which groups are distinct species. d. In the study of evolution, the species is the unit over which change is measured. 34. If a population stopped reproducing sexually, but still reproduced asexually, how would its genetic variation be affected over time? Could speciation occur in this situation? Explain your ideas. Figure 18.26 37. Which type of speciation, allopatric or sympatric, is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 753 more common? Why? a. Allopatric speciation is more common because it a. Separate species cannot inter |
breed, so hybrid reproduction does not occur in nature prevents gene flow between the species. b. b. Allopatric speciation is more common because it involves stronger prezygotic barriers. c. Sympatric speciation is more common because it prevents gene flow between the species. d. Sympatric speciation is more common because it involves stronger prezygotic barriers. 38. Use adaptive radiation to explain the diversification of the finches Darwin observed in the Galapagos. a. The finches likely shared a common ancestor when they came to the island, but exhibited different traits. Each species of finch settled the island where its particular traits would be the most adaptive. b. The finches likely originated as one parent species, but over time mutations caused them to develop reproductive barriers and separate into different species. To reduce competition, the species then radiated out to inhabit different islands. c. The finches likely dispersed from one parent species, and natural selection based on different food sources in differing habitats led to adaptive changes, evidenced in the different beak shapes of the different species-each suited to a different food type. d. It is likely that a series of cataclysmic events caused an original finch species to diverge into the many finch species that inhabited the islands when Darwin observed them. The different species then radiated out to the different islands and adapted to the different conditions on each. 39. Describe a situation where hybrid reproduction would cause two species to fuse into one. If the hybrid offspring are more fit than the parents, reproduction would likely continue between both species and the hybrids, eventually bringing all organisms under the umbrella of one species c. Two species that have recently diverged from each other can reproduce with each other, creating hybrid individuals that belong to the species of the parents’ common ancestor. d. If two species occupy the same niche in the same area, they can either compete or they can collaborate and reproduce with each other, eventually fusing into a single species 40. What do both rate of speciation models have in common? Explain. a. Both models ignore the influence of gene flow for simplicity’s sake. b. Both models apply only to island chains. c. Both models require the influence of cataclysmic events which precipitate rapid adaptation and speciation d. Both models conform to the rules of natural selection and the influences of gene flow, genetic drift, and mutation 41. Describe a situation where hybrid reproduction would cause two species to |
continue divergence. a. b. c. d. f two closely related species continue to produce hybrids, the hybrids will compete with both species, causing them to find new niches which will further their divergence If two closely related species continue to produce hybrids, they will develop reproductive barriers to prevent production of hybrids, to ensure that they remain separate species. If two closely related species continue to produce hybrids that are less fit than the parent species, there would be reinforcement of divergence. f two closely related species continue to produce hybrids they will always converge into a single species TEST PREP FOR AP® COURSES 42. Prior to 1800 in England, the typical moth of the species Biston betularia (peppered moth) had a light pattern. Dark colored moths were rare. By the late 19th century, the light-colored moths were rare, and the moths with dark patterns were abundant. The cause of this change was hypothesized to be selective predation by birds (J.W. Tutt, 1896). During the industrial revolution, soot and other wastes from industrial processes killed tree lichens and darkened tree trunks. Thus, prior to the pollution of the industrial revolution, dark moths stood out on light-colored trees and were vulnerable to predators. With the rise of pollution, however, the coloring of moths vulnerable to predators changed to light. Which of the following aspects of Darwin’s theory of 754 Chapter 18 | Evolution and Origin of Species evolution does the story of the peppered moth most clearly illustrate? pool is demonstrated by the story, but the peppered moth stays a peppered moth. a. There is competition for resources in an overbred population. Which scenario, if it were to occur, would be a model for large-scale evolutionary change? a. Conditions change such that the dark form of the moth is favored and the light form is diminished in the population due to predation. Conditions change again, the dark form is vulnerable, and the light form returns to prevalence. b. Conditions change such that the dark form of the moth is favored and the light form is eradicated in the population due to predation. Conditions change again, the dark form is vulnerable, and the dark form is eradicated due to predation. c. Conditions change such that dark form of the moth is favored and the light form is diminished in the population due to predation. Conditions change again, and both forms have equal prevalence. d. Conditions change such that dark form of the moth |
is favored and the light form is eradicated in the population due to predatio<|endoftext|>n. Conditions change again, the dark form is vulnerable. It develops an adaptation that shields it from predation. 45. Given your understanding of evolutionary theory and the relationship between evolution and the genetic makeup of populations, which statement is false? a. Homologous characteristics that have evolved more recently are shared only within smaller groups of organisms. b. The genetic code is a homologous characteristic shared by all species because they share a common ancestor in the deep past. c. DNA sequence data would likely support any evolutionary tree drawn from anatomical data sets. d. The degree of relatedness between groups of organisms is only sometimes reflected in the similarity of their DNA sequences. 46. Each of the following observations comes from a different scientific discipline. Which is the best support for Darwin’s concept of descent with modification? b. There is great variability among members of a population. c. There is differential reproduction of individuals with favorable traits. d. The majority of characteristics of organisms are inherited. 43. Prior to 1800 in England, the typical moth of the species Biston betularia (peppered moth) had a light pattern. Dark colored moths were rare. By the late 19th century, the light-colored moths were rare, and the moths with dark patterns were abundant. The cause of this change was hypothesized to be selective predation by birds (J.W. Tutt, 1896). During the industrial revolution, soot and other wastes from industrial processes killed tree lichens and darkened tree trunks. Thus, prior to the pollution of the industrial revolution, dark moths stood out on light-colored trees and were vulnerable to predators. With the rise of pollution, however, the coloring of moths vulnerable to predators changed to light. In the late 1900s, England cleaned up its air, and pollution decreased. The bark of trees went from dark to light. Which of the following outcomes to the populations of peppered moth would you expect given this environmental change? a. An increase in the number of dark moths and a decrease in the number of light moths b. an increase in the number of moths overall c. an approximately equal number of light moths and dark moths d. an increase in the number of light moths and a decrease in the number of dark moths 44. Prior to 1800 in England, the typical moth of the species Biston betular |
ia (peppered moth) had a light pattern. Dark colored moths were rare. By the late 19th century, the light-colored moths were rare, and the moths with dark patterns were abundant. The cause of this change was hypothesized to be selective predation by birds (J.W. Tutt, 1896). During the industrial revolution, soot and other wastes from industrial processes killed tree lichens and darkened tree trunks. Thus, prior to the pollution of the industrial revolution, dark moths stood out on light-colored trees and were vulnerable to predators. With the rise of pollution, however, the coloring of moths vulnerable to predators changed to light. Commonly used in biology textbooks, the peppered moth is a classic example of evolutionary change in action. The example describes changes in a population’s allele frequencies-a small-scale change, evolutionarily speaking. The presence of both light and dark forms within the gene This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 755 a. The process of mutation, which generates genetic variation, is random. However, the process of natural selection, which results in adaptations like the fit between a flower and its pollinator, favors variants which are better able to survive and reproduce. Natural selection is not random, so the overall process of evolution is not random, either. b. The process of mutation, which generates genetic variation, is random. However, the process of migration, which results in gene flow between populations, also generates genetic variation. Migration is not random, so the overall process of evolution is not random, either. c. The process of mutation, which generates genetic variation, is random. However, the process of sexual reproduction, which also introduces genetic variance, is not random. Because sexual reproduction is not random, the overall process of evolution is not random, either. d. The process of mutation, which generates genetic variation, is random. Whether mutations have a positive, negative, or neutral effect in terms of selective advantage is also random. Mutations and their effects are random, so the overall process of evolution is random. 50. The selective breeding of plants and animals that possess desired traits is a process called artificial selection. For example, broccoli, cabbage, and kale are all vegetables that have been selected from one species of wild mustard. How is artificial selection both similar to and different from Darwin’s conception |
of natural selection? Does artificial selection provide evidence for evolution by natural selection? Explain. a. Geologists provide evidence that earthquakes reshape life by causing mass extinctions. b. Botanists provide evidence that South American temperate plants have more in common with South American tropical plants than temperate plants from Europe. c. Zoologists provide evidence that fewer animal species live on islands than on nearby mainlands. d. Ecologists provide evidence that species diversity increases closer to the equator. 47. Paleontologists have recovered a fossil for an organisms named Archaeopteryx. It has many features in common with reptiles, but, like birds, shows evidence of feathers. For what aspect of evolutionary theory does this piece of evidence suggest support? a. Modern species are distinct natural entities. b. Modern species are not currently evolving. c. Modern species share a common ancestor. d. Modern species have both convergent and divergent traits. 48. Which of the following pieces of evidence illustrates evolution as an ongoing process? a. Some genes from the bacterium E. coli have sequences that are similar to genes found in humans. b. Marsupial mammals live in just a few places in the world today-Australia, South America, and part of North America. c. The fossil record shows that Rodhocetus, an aquatic mammal related to whales, had a type of ankle bone that is otherwise unique to a group of land animals. d. In the 1940s, infections by the bacterium Staphylococcus aureus could be treated with penicillin; today populations exist that are completely resistant. 49. The process of mutation, which generates genetic variation, is random. Thus, life has evolved, and continues to evolve, randomly. Which statement is an appropriately evidence-based refinement of the above? 756 Chapter 18 | Evolution and Origin of Species a. Both artificial selection and natural selection are the differential reproduction of individual organisms with favored traits. In artificial selection, humans have actively modified plants and animals by selecting and breeding individuals with traits deemed desirable. In natural selection, the most successful individuals in a species are selected by the species to reproduce b. Both artificial selection and natural selection are processes that result in better-adapted individuals within a species. In artificial selection, humans have actively modified plants and animals by selecting beneficial genes from other organisms and inserting them into the target organisms. In natural selection, natural processes such as mutations and viruses introduce new genes to a population c. Both artificial selection and natural selection are processes |
that cause organisms to be better adapted over time. In artificial selection, humans have trained animals to be more successful in completing tasks that the humans want completed. In natural selection, organisms train the functions that they will need to survive and reproduce d. Both artificial selection and natural selection are the differential reproduction of individual organisms with favored traits. In artificial selection, humans have actively modified plants and animals by selecting and breeding individuals with traits deemed desirable. In natural selection, individuals are selected naturally as its traits deem it more fit for survival and reproduction 51. Genes important in the embryonic development of animals have been relatively well conserved during evolution. This means they are more similar among different species than many other genes. What explains this genetic conservation across animal species? a. Changes in the genes that are important to embryonic development have been relatively minor because there are no selective pressures on an individual before it is born b. Changes in the genes that are important to embryonic development have been relatively minor because not much time has elapsed since the divergence of the various animal taxa. c. Changes in the genes that are important to embryonic development have been relatively minor because early embryos are very fragile and even small mutations can result in death d. Changes in the genes that are important to embryonic development have been relatively minor because mutational tweaking in the embryo has magnified consequences in the adult This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 52. The upper forelimbs of humans and cats have fairly similar structures. In contrast, the upper forelimbs of whales (their flippers) have bones with a different shape and proportion from both cats and humans. Interestingly, genetic data suggests that all three organisms have a common ancestor from about the same point in time. What is a likely explanation for these data? a. Cats and humans are more closely related to each other than either are to whales. b. The shape of the whale forelimb arose a result of disadvantageous mutations c. The whale flipper is an adaptive characteristic unique to its water environment. d. The whale flipper is a vestigial structure. 53. Biogeography is the study of biological species as they relate to geographical space and geological time. The fossil record shows that dinosaurs originated about 200 to 250 million years ago. Would you expect the geographic distribution of early dinosaur fossils to be broad (on many continents) or narrow (on one or a few continents)? Explain. a. broad because dinosaurs originated before the breakup |
of Pangaea b. broad because some dinosaurs could fly between continents c. narrow because they went extinct too quickly to disperse very far d. narrow because they lived so long ago that the fossils have mostly broken down or disappeared 54. The term microevolution describes evolution on its smallest scale: the change in allele frequencies in a population over generations. DDT is a pesticide that was widely in use in the United States from the 1940s until 1972. The table below summarizes a particular allele frequency in laboratory strains of the common fruit fly, Drosophila melanogaster. Strains collected from flies in the wild in the 1930s Strains collected from flies in the wild in the 1960s 0% 40% Frequency of allele conferring DDT resistance Using this information, describe a model in which natural selection improved the match between D. mealanogaster and its environment through microevolution. Chapter 18 | Evolution and Origin of Species 757 a. DDT killed off a large proportion of the population, and the alleles present in the surviving fruit flies differed from those in the original population b. Mutations from the application of DDT caused the allele conferring DDT resistance to appear in the population. c. Female mosquitoes chose to mate with male mosquitoes that had the allele conferring DDT resistance because it would make their offspring more fit. d. The wide use of DDT meant that fruit flies with DDT resistance were more evolutionarily fit than their counterparts without DDT resistance. 55. In 1795, a Scottish geologist named Charles Hutton suggested that Earth’s geologic features could be explained by gradual processes that were still operating. This was in direct contrast to other scientific thought at the time, which included well-accepted proposals that geologic layers were representative of catastrophic events caused by processes no longer operating in the present time. Hutton proposed geologic features as the result of slow and consistent change, such as valleys formed by rivers wearing through rock. Hutton’s ideas were incorporated in the work of Charles Lyell, a geologist working in Darwin’s time. Lyell advocated a principle called uniformitarianism, the consistency of mechanisms of change over time. In other words, Lyell argued that the same geologic processes operating in the present had operated in the past, and at the same rate. The ideas of Hutton and Lyell influenced the work of Charles Darwin. How do Hutton’s and Lyell’s ideas connect to and provide support for |
Darwin’s theory of evolutionary change? a. The idea that the same processes that operate in the present also operated in the past, and at the same rate, supported Darwin’s hypothesis of natural selection because humans could select for desirable traits and produce change very rapidly, so natural selection would also be fast enough to produce the full range of diversity in living organisms. b. The idea that the same processes that operate in the present also operated in the past, and at the same rate, connects to Darwin’s hypothesis of natural selection because he had observed it happening in the present c. The idea that geologic change is the result of slow, continuous processes rather than sudden, substantial change connects to Darwin’s support of gradualism rather than punctuated equilibrium as the process that guided evolution. d. The idea that geologic change is the result of slow, continuous processes rather than sudden, substantial change connects directly to Darwin’s hypothesis that, given enough time, slow and subtle processes could produce substantial biological change. 56. The human immunodeficiency virus (HIV) reproduces very quickly. A single virus can replicate itself a billion times in one 24-hour period. In a hypothetical treatment situation, a patient’s HIV population consists entirely of drug-resistant viruses after just a few weeks of treatment. How can this treatment result best be explained? How does this explanation illustrate that evolution is an ongoing process? a. The resistant viruses passed their genes to the non-resistant viruses so that 100% of the viruses became resistant. This illustrates evolution as an ongoing process because the genes of the population changed in real time. b. The non-resistant viruses died, and the resistant ones survived and rapidly reproduced. This illustrates evolution as an ongoing process because the change in the HIV population is the result of natural selection. c. The viruses developed resistance to the drug after repeated exposure to it. This illustrates evolution as an ongoing process because the viruses were able to adapt to changing conditions. d. The drug-resistant viruses were more fit than their non-resistant counterparts to begin with, and over time they dominated the population. This illustrates evolution as an ongoing process because natural selection favored one phenotype over another. 57. A friend says: “Natural selection is about the survival of the very fittest in a population. The fittest are those that 758 Chapter 18 | Evolution and Origin of Species are strongest, largest, fastest.” Would you agree with that statement? Explain. What evidence from scientific |
disciplines can you offer to support your agreement or your disagreement? a. The statement is true. If an organism is not strong and fast, it will not survive long enough to reproduce and pass on its genes, and if it is not large and fitter than the other individuals around it then it will not be able to compete for a mate. Many seal species, for example, have only a single male who gets to mate. He must be the very fittest seal to win all the females. b. The very fittest organisms are not necessarily the ones that survive. Sometimes it is the least fit organisms that survive and reproduce. For example, in one generation the mice who are bad at foraging for seeds may reproduce prolifically and dominate the mice who are good at foraging. In this case, natural selection will select for the less-fit phenotype and spread it in the population. c. The definition of fitness is not correct. The strongest and fastest organisms are more fit than the weaker and slower ones, but large individuals are often at a disadvantage to smaller ones because they are easily spotted by predators. For example, a large rabbit will stick out on a field more than a small one and will get eaten by a hawk. d. What is meant by “fittest” is not necessarily strong, large, and fast. Fitness, as defined in evolutionary terms, has to do with survival and the reproduction of genetic material. For example, a small but showy male bird may be selected by female birds to reproduce, while a large but less colorful one is not. 58. A student placed 20 tobacco seeds of the same species on moist paper towels in each of two petri dishes. Dish A was wrapped completely in an opaque cover to exclude all light. Dish B was not wrapped. The dishes were placed equidistant from a light source set to a cycle of 14 hours of light and 10 hours of dark. All other conditions were the same for both dishes. The dishes were examined after 7 days, and the opaque cover was permanently removed from dish A. Both dishes were returned to the light and examined again at 14 days. The following data were obtained: This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Figure 18.27 Which of the following best supports the hypothesis that the difference in leaf color is genetically controlled? a. b. c. d. the number of yellow-leaved seedlings in dish A on day 7 |
the number of germinated seeds in dish A on days 7 and 14 the death of all the yellow-leaved seedings the existence of yellow-leaved seedlings as well as green-leaved ones on day 14 in dish B 59. Use the data from Figure 18.27 to answer the question. Which best describes the usefulness of the yellow-leaved phenotype as a variation subject to natural selection? Chapter 18 | Evolution and Origin of Species 759 a. b. c. d. the yellow-leaved phenotype can germinate in environments without light the germination of the yellow-leaved phenotype is unaffected by light intensity the germination of the yellow-leaved phenotype is accelerated as compared to the green-leaved phenotype the yellow-leaved phenotype cannot germinate in environments with light 60. Use the data from Figure 18.27 to answer the question. Yellow-leaved seedlings are unable to convert light energy to chemical energy. Which observation is most likely to be made on day 21? a. searching horizontal rock layers in any class of rock and trying to find those that contain the greatest number of fossils b. collecting fossils from rock layers deposited prior to the Permian period that contain some early vertebrate bones c. looking in sedimentary layers next to bodies of water in order to find marine fossils of bivalves and trilobites d. using relative dating techniques to determine the geological ages of the fossils found so they can calculate the rate of speciation of early organisms a. a few yellow-leaved seedlings alive in dish A, but none in dish B b. a few yellow-leaved seedlings alive in dish B, but none in dish A c. no yellow-leaved seedlings alive in dish A or dish B d. a few yellow-leaved seedlings alive in dish A and dish B 63. Populations of a plant species have been found growing in the mountains at altitudes above 2,500 meters. Populations of a plant that appears similar, with slight differences, have been found in the same mountains at altitudes below 2,300 meters. Describe a plan for collecting two kinds of data that could provide a direct answer to the question: do the populations growing above 2,500 meters and the populations growing below 2,300 meters represent a single species? 61. Populations of a nocturnal toad live along a long river. On the other side of a band of territory that is about 10 kilometers wide, there |
are populations of a toad that appear similar. Which of the following data would provide compelling evidence that the two populations represent different species? a. The populations of toads on the other side of the banded territory are not completely nocturnal. b. Fertile hybrid populations of toads are found between the two other populations. c. There appear to be some hybrid toads between the two populations, but they are few and frail. d. The two populations of toads enact very different mating behaviors. 62. > a. Scientists could take the genetic code of a plant from each altitude and determine whether the two sets of DNA are identical. They could also insert genes from one plant into the cells from the other and see if the cells survive b. Scientists could look in the fossil record to find the plants’ most recent common ancestor. They could also check the surrounding mountains to determine if the most recent common ancestor is still living. c. Scientists could breed the two groups in the same environment and observe whether, over several generations, they begin to look more similar. They could also switch the groups, growing the high-altitude plants at low altitude and the low-altitude plants at high altitude, and observe whether the former begin to look like low-altitude plants and the latter begin to look like high-altitude plants. d. Scientists could collect seeds and test whether they might be cross-pollinated to produce fertile offspring. They could also investigate the area between 2,500 meters and 2,300 meters to see if fertile hybrid populations might be found living between the two other populations of plants. 64. Populations of a plant species have been found growing in the mountains at altitudes above 2,500 meters. Populations of a plant that appears similar, with slight differences, have been found in the same mountains at altitudes below 2,300 meters. Explain how the two types of data you suggested provide a direct answer to the question of whether speciation has 760 taken place. a. b. c. d. If the plants become more similar when grown in the same environment, or if the high-altitude plants respond to low altitude in the same way that low-altitude plants have, and low-altitude plants respond to high altitude the same way that high-altitude plants have, then the two groups have the same underlying genetic structure and belong to one species. If the seeds from the plants can be cross fertilized and developed into fertile offspring, the two populations are not yet |
reproductively isolated and remain one species. If hybrid forms are found, the two populations are not reproductively isolated and hybrids are both viable and successful. If the genetic codes of the two plants are identical, then they must belong to the same species. Also, if genes transplanted between the plants function successfully, then the plants must be similar enough to each other to belong to the same species. If scientists are able to find the common ancestor of the two groups in the fossil record or in neighboring communities, then they can determine whether the plants have diverged into separate species or remain a single species. 65. Assuming a population that has genetic variation and is under the influence of natural selection, place the following events in the order in which they would occur: • Genetic frequencies within the population change. • A change occurs in the population’s environment. • Phenotypic variations shift. • • Individuals who are well-adapted leave more offspring than individuals who are poorly adapted. Individuals who are poorly adapted do not survive at the same rate as individuals who are well adapted. Chapter 18 | Evolution and Origin of Species a. 1. A change occurs in the population’s environment. 2. 3. Individuals who are poorly adapted do not survive at the same rate as individuals who are well adapted. Individuals who are well-adapted leave more offspring than individuals who are poorly adapted. 4. Genetic frequencies within the population change. 5. Phenotypic variations shift. b. 1. A change occurs in the population’s environment. 2. Genetic frequencies within the population change. 3. Phenotypic variations shift. 4. 5. Individuals who are poorly adapted do not survive at the same rate as individuals who are well adapted. Individuals who are well-adapted leave more offspring than individuals who are poorly adapted. c. 1. Phenotypic variations shift. 2. A change occurs in the population’s environment. 3. Genetic frequencies within the population change. 4. 5. Individuals who are poorly adapted do not survive at the same rate as individuals who are well adapted. Individuals who are well-adapted leave more offspring than individuals who are poorly adapted. d. 1. Individuals who are well-adapted leave more offspring than individuals who are poorly adapted. 2. Individuals who are poorly adapted do not survive at the same rate as individuals who are well adapted. 3. Phenotypic variations shift. 4. Genetic frequencies within the population change. 5. A change occurs in the population’ |
s environment. 66. A biologist studies a population of voles for 20 years. During almost the entire research period, the population stays between 50 and 75 individuals. Additionally, fewer than half of the voles born do not survive to reproduce, due to predation and competition for food. Then, in one generation, 80% of the voles born live to reproduce. The population increases to 110 individuals. What inferences This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 761 about food and predation can you make for the singular generation in which 80% of offspring survived? What prediction can you make about the genetic and phenotypic variation of future populations for this group of voles? a. Either there was fewer food available or the degree of predation increased. The future generations of this group of voles should evidence fewer genetic variation. b. Either there was fewer food available or the degree of predation increased. The future generations of this group of voles should evidence greater genetic variation. c. Either there was more food available or the degree of predation decreased. The future generations of this group of voles should evidence less genetic variation. d. Either there was more food available or the degree of predation decreased. The future generations of this group of voles should evidence greater genetic variation. 67. There are years of drought in a small, relatively isolated community. During the drought, small seeds with thin shells become rare. Large seeds with hard cases become increasingly common. The large, tough seeds are successfully eaten by birds with large and broad beaks. Assuming that the drought continues and the population of birds in the community stays isolated, what predictions for the population can you make under the influence of natural selection? a. The birds with small, thin beaks will grow larger, broader beaks to be able to eat the larger seeds. This will result in subsequent generations having a higher percentage of birds with large, broad beaks. b. There will be more birds with small, thin beaks dying and more birds with large, broad beaks surviving. Differential reproduction of birds with large, broad beaks will result in subsequent generations having a higher percentage of birds with large, broad beaks. c. The species will diverge into two species, one with small, thin beaks and one with large, broad beaks. The two species will then compete for resources. d. There |
will be neither phenotypic nor genotypic changes in the population. 68. At one time, avian researchers in the Sulawesi region of Indonesia described the Flowerpecker populations on the mainland and the Wakatobi archipelago as one species. A recent reassessment of the Wakatobi populations resulted in the suggested reclassification of these populations as a distinct species, the Wakatobi Flowerpecker. Which of the following pieces of evidence, if true, would be cause for this reclassification? a. The populations have become dependent on the island food sources. b. The populations have become morphologically distinct from the mainland species. c. The populations have become adapted to the island habitat. d. The populations have become reproductively isolated from the mainland species. 69. What pattern in the fossil record would you expect to see to support the model of gradual speciation? How would you expect this pattern to differ from a pattern in the fossil record that supports the model of punctuated equilibrium? Explain. a. b. c. In the case of gradual speciation, the fossil record would show only a few hybrid individuals, followed by individuals of the two distinct species. For the case of punctuated equilibrium, the fossil record would show many hybrid individuals persisting through several geological layers. In the case of gradual speciation, the fossil record would show the parent species in a single location, such that the newly diverged species remained in contact with each other. For the case of punctuated equilibrium, the fossil record would show a geographic divide within the parent species that caused it to diverge into multiple new species. In the case of gradual speciation, the fossil record would show many intermediate forms. For the case of punctuated equilibrium, the fossil record would show new forms that persist essentially unchanged through several geological layers, then disappear just as a new form appears. d. Gradual speciation would be undetectable in the fossil record. For the case of punctuated equilibrium, the fossil record would show a steady progression of distinct forms. 70. Until recently, these three species of short-tailed pythons, Python curtus, Python brongersmai (middle), and Python breitensteini were considered one species. However, due to the different locations in which they are found, they have become three distinct species. What is this an example of? a. divergent evolution b. sympatric speciation c. allopatric speciation d. variation |
71. Consider two species of birds that diverged while separated geographically but resumed their contact before reproductive isolation was complete. Which describes the first step in what would happen over time if the two 762 Chapter 18 | Evolution and Origin of Species species mated extensively and their hybrid offspring survived and reproduced more poorly than offspring from intra-species matings? a. Natural selection would cause prezygotic barriers to reproduction between the parent species to strengthen over time. b. The production of unfit hybrids would increase and the speciation process would complete. c. The extensive mating between the species would continue to produce large numbers of hybrids. d. The gene pools of the parent species would fuse over time, reversing the speciation process. SCIENCE PRACTICE CHALLENGE QUESTIONS stromatolites and the age of the oldest exposed rock to show how evidence from different scientific disciplines provides support for the concept of evolution. Evaluate the legitimacy of claims drawn from these different disciplines (biology, geology, and mathematics) regarding the origin of life on Earth. 72. In addition to biology, evidence drawn from many different disciplines, including chemistry, geology, and mathematics, supports models of the origin of life on Earth. In order to determine when the first forms of life likely formed, the rate of radioactive decay can be used to determine the age of the oldest rocks (see optional problems C and D, below) exposed on Earth’s surface. These are found to be approximately 3.5 billion years old. The age of rocks can be correlated to fossils of the earliest forms of life. A. The graph compares times of divergence from the last common ancestor based on the fossil record with a "molecular time" constructed by comparing sequences of conserved proteins to determine a mutation rate (after Hedges and Kumar, Trends in Genetics, 2003). Explain how such a molecular clock could be refined to infer time for the evolution of prokaryotes. Figure 18.29 The oldest known rocks are exposed at three locations: Greenland, Australia, and Swaziland. The following application of mathematical methods provides the essential evidence of the minimum age of Earth. The mathematics is appropriate for students who have completed a second year of algebra. However, it is not illustrative of the type of item that could appear on the AP Biology Exam. The exposed rocks contain a radioactive isotope of rubidium, 87Rb, which decays into a stable isotope of strontium, 87Sr. An 87Rb atom with 37 |
protons and 50 neutrons decays when a proton is converted into a neutron to produce an atom, 87Sr, with 36 protons and 51 neutrons. As time passed, the number of each isotope changed from its initial value. When a crystal containing 87Rb atoms formed from the molten surface of the hot, early Earth during the Hadean eon, the number of these atoms at that initial time can be represented as N87Rb,0. As time passed, the number of atoms of this isotope changed to N87Rb. C. Justify the relationship between the number of each isotope at any time and the number of each at the time that the molten rock solidified (denoted by the subscript 0): N87Sr = N87Sr, 0 + N87Rb, 0 − N87Rb Figure 18.28 B. Using a molecular clock constructed from 32 conserved proteins, Hedges and colleagues (Battistuzzi et al., BMC Evol. Biol. 2004) estimated the times during which key biological processes evolved. A diagram based on their work is shown. Connect the time of the origin of life inferred from this diagram with the age of the oldest fossil This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 763 The decay of unstable radioisotopes is exponential with a half-life of T1/2, which for 87Rb is 4.88 × 1010 years: − 0.693t/ T1 / N87Rb = N87Rb, 0 This can be used to replace the initial number of 87Rb atoms, which cannot be measured, with the present-day value: e 2 = + (e 2 − 1) N87sr, 0 N86sr N87Rb N86sr − 0.693t/ T1 / N87sr N86sr When the measurements of the numbers of 87Rb and 87Sr were made (Moorbath et al., Nature, 1972), measurements of a second stable isotope of strontium, 86Sr, were also made. The ratio of the initial number of 87Sr and 86Sr atoms is the same as today, since the isotopes are both stable. The value of this ratio is 0.71. This is a linear equation in the form y = ax + |
b, where a is the term in parenthesis containing the half-life of 87Rb. If Y = N87Sr/ N86Sr is graphed versus N87Rb/ N86Sr, the slope can be used to determine the time, t, that has passed since the rock formed from melting: 0.683t/ T1 / a = e t = ln(a + 1) • T1 / 2 − 1, so 2 / 0.693 D. Data on the rubidium and strontium isotopes at Isua in Greenland are provided in the table. Analyze these data to obtain the age of formation of these rocks. N 87Rb / N 86Sr N 87Sr / N 86Sr.212.214.223.259.268.267.290.394.434 Table 18.1.711.711.712.714.714.715.716.720.723 The solidification of the molten surface of Earth at the end of the Hadean eon (4 to 4.6 billion years ago) and the condensation of liquid oceans provided a medium from which life emerged. The most ancient fossils are colonial, photosynthetic cyanobacteria called stromatolites. As climate change melted the perennial snow covering Greenland, new geologic evidence of the time of that origin was obtained (Nutman et al., Nature 2016) with the discovery of the most ancient stromatolites. These fossils record communities of photosynthetic bacteria embedded in Isua sediments 3.7 billion years ago. Worldwide stromatolite fossils show a decline between 1 and 1.3 billion years ago. 73. In 1952, the Miller-Urey experiment showed that an electrical discharge in a gas-phase mixture of ammonia, hydrogen, methane, and water produced five amino acids. When the experiment was conducted, evidence indicated that this mixture was representative of the Hadean (early Earth) atmosphere. The experiment was repeated in the presence of jets of hot steam, simulating Hadean volcanic eruptions and producing an even larger variety of amino acids. A. Consider the following criticisms of the “organic soup” model and justify the selection of data that other experiments might provide regarding the origin of life on Earth. • Biopolymers on Earth have a left-hand symmetry at the carbon adjacent to the carboxylic acid carbon, and these experiments produced mixtures of both |
leftand right-hand symmetries. • No peptide bonds between amino acids were observed. • Early Earth’s atmospheric oxygen concentration is known to have been very low, implying the absence of an ozone layer to filter high-energy ultraviolet (uv) radiation. • Ammonia decomposes when it absorbs high-energy uv radiation, but diatomic nitrogen does not. Models of the abiotic synthesis of biomolecules suffer from a “chicken and egg” dilemma. Proteins are needed to synthesize DNA and RNA, and DNA and RNA are needed to synthesize proteins. Which molecules came first? B. In light of the following observations, evaluate the hypothesis that nucleotides arose from a prebiotic mixture. • Nuclei acids are not found in experiments like those of Miller and Urey. • Purines and pyrimidines decompose at high temperature, and Earth was bombarded by meteors and comets during the Hadean eon. • Bonds in the purine and pyrimidine rings of nucleic acids are broken by high-energy uv radiation. • Carl Sagan and colleagues synthesized ATP from a mixture of adenosine, ribose, and phosphate when exposed to uv radiation. • Ribose has never been synthesized in experiments like those conducted by Miller and Urey. • Ribose has a left/right symmetry, and the right- handed form occurs in Earth organisms. Continuing with the analogy, if neither the chicken nor the egg came first, then both must have arisen together. Some 764 Chapter 18 | Evolution and Origin of Species regard simultaneous innovations in both catalysis and information storage and retrieval as too improbable. In samples of meteorites, both amino acids and nucleic acids have been found. The amino acids are mixtures of left- and right-handed symmetries, although some have shown a significant bias toward the left-handed form (J. Elisa et al., ACS Central Science, 2016). The arrival from space of the seeds of biomolecules is called panspermia. Carl Sagan (1966) and Francis Crick (1973), one of the first to describe the structure of DNA, regarded panspermia as the only plausible origin of life on Earth. In fact, their belief was in directed panspermia, the intentional seeding by intelligent aliens. C. Describe the questions that must be addressed for panspermia to be a scientific hypothesis about the origin of life on Earth and describe the reasons |
for the directed panspermia revision of this hypothesis. To avoid the conflicting chicken-and-egg claims that “protein catalyst was first” and “DNA information storage was first,” two alternatives have emerged regarding the origin of life on Earth. Consider two simple ideas: 1) water blocks uv radiation, and cracks in the ocean floor (hot vents) provide a temperature difference that generates a source of entropy; and 2) ribosomes are composed of RNA. D. Describe one of the following as a hypothesis concerning the origin of life on Earth: • Reactions among molecules in the vicinity of hot vents became organized in space and time, eventually developing structures that foreshadow the proton gradient upon which metabolism is based. This alternative is the basis for what is referred to as the metabolism-first hypothesis. • The catalytic properties of the ribosome reflect the self-catalytic polymerization of nucleotides with sequential structures conserved in modern DNA, the catalytic properties conserved in proteins, and the catalytic properties of the ribosome whose core structure is RNA. This alternative is the basis for what is referred to as the RNA-first hypothesis. 74. The radiant energy emitted by a star gradually increases after its birth. During the Hadean eon, while the molten Earth cooled and life emerged, the Sun provided approximately 25% less radiant energy than it does now. Ignoring effects due to differences in the composition of Earth's atmosphere between then and now, this means that the average surface temperature of the surface would be about 25 °C below the freezing temperature of water. Evidence of liquid water on Earth during the Hadean eon is provided by geologic structures known only to form in liquid water, such as lava pillows and the stromatolites that are the fossilized layers of photosynthetic cyanobacteria. Pose a scientific question that guides inquiry into early Earth conditions that supported the innovation of photosynthesis. 75. Connect the techniques of radiometric measurement, anatomy, and molecular biology to the supporting evidence of the theory of evolution provided. 76. Describe reasons for the revision of scientific hypotheses of the origin of life on Earth. 77. Directed evolution is an inquiry strategy that is usually used to investigate gene expression or the function of proteins that are expressed. The investigator imposes a selection pressure and observes the evolution of a population. In one investigation, unicellular yeast were allowed to sediment in a column of a nutrient-containing solution. Individuals that |
traveled furthest towards the bottom of the column were removed and placed in a new column. After 60 generations of repeated selection, yeast became multicellular. In this experiment, selection was acting on the collection of cells and not on the individual. To test the claim that selection was acting on the multicellular system and not just individual cells, the investigators compared the effects on a population of yeast that had acquired multicellularity by strong selection (allowing only 5 minutes to settle) and weak selection (allowing 25 minutes to settle). A strong selection increased cluster size, and a weak selection decreased cluster size. A. Evaluate the claim that the use of both a strong and weak selection demonstrates that evolution is an ongoing process that, under artificially imposed conditions, led to the emergence of multicellularity in a single-celled organism. B. In this directed evolution study, the selection pressure imposed by the investigators led to a new phenotype. Consider a situation in which there is a vertical variation in the density of nutritional resources. Analyze the advantages and disadvantages of cooperative behavior, including changes in the likelihood of replication of the individual and population genomes. 78. Selection processes in changing and unchanging environments differ. Connect the effects of negative and positive selection pressures to changes in the environment. 79. In biology, the word “race” is rarely used. It could be imagined to be synonymous with a subspecies. Species is well defined, at least when horizontal gene transfer is not taken into account, by reproductive isolation. Speciation may arise through geographic isolation. A. Aside from geographic isolation leading to reproductive isolation, predict two other mechanisms of speciation in a population and how these mechanisms can lead to a scientific definition of a subspecies. The use of the term “race” with regard to human populations might be a reference to cultural or socioeconomic isolation, and has often been mistaken to have biological significance. Rosenberg et al. (Science, 2002) sampled the genes of 1,056 people from 52 populations. They compared genetic variations within each population to variations among populations. They found that differences between individuals in two different This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 18 | Evolution and Origin of Species 765 populations were, on average, roughly 20 times smaller than differences between two individuals in the same population. B. Groups of humans have often been geographically isolated for long periods of time until isolation is broken by invasion, ensl |
avement, migration, or another similar event. Invaders have traditionally been male. Predict the effect of invasion on the differential inheritance of genes in X- and autosomal chromosomes. 766 Chapter 18 | Evolution and Origin of Species This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 767 19 | THE EVOLUTION OF POPULATIONS Figure 19.1 Living things may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution. (credit "wolf": modification of work by Gary Kramer; credit "coral": modification of work by William Harrigan, NOAA; credit "river": modification of work by Vojtěch Dostál; credit "fish" modification of work by Christian Mehlführer; credit "mushroom": modification of work by Cory Zanker; credit "tree": modification of work by Joseph Kranak; credit "bee": modification of work by Cory Zanker) Chapter Outline 19.1: Population Evolution 19.2: Population Genetics 19.3: Adaptive Evolution Introduction Evolutionary medicine is an emerging field that applies evolutionary theory to modern medicine. Rather than just seeking answers to how illness occurs, evolutionary medicine also asks why illness occurs. This approach to medicine has led to many important advances. For example, endogenous retroviruses (ERVs) are pieces of retroviruses that began invading mammalian genomes over 100 million years ago. While studying why smaller mammals tend to get cancer more frequently than larger mammals, scientists discovered that larger mammals have had fewer ERVs invade their genome. Because retroviral integration is associated with cancer, results from this research suggest the possibility that larger mammals are able to control EVR replication until they reach post-reproductive age. More on this research can be found on the PLOS Pathogens website (http://openstaxcollege.org/l/32mammalcancer). [1] 1. Katzourakis A, Magiorkinis G, Lim AG, Gupta S, Belshaw R, et al. (2014) Larger Mammalian Body Size Leads to Lower Retroviral Activity. PLoS Pathog10(7): e1004214. doi: 10.1371/journal.ppat.1004214 768 Chapter 19 | The Evolution of Pop |
ulations 19.1 | Population Evolution In this section, you will explore the following questions: • What is population genetics and how is population genetics a synthesis of Mendelian inheritance and Darwinian evolution? • What is the Hardy–Weinberg principle, and how can it be applied to microevolution? The mechanisms of inheritance, or genetics, were not understood at the time Charles Darwin and Alfred Russel Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to understanding many aspects of evolution. In fact, the predominant (and incorrect) genetic theory of the time, blending inheritance, made it difficult to understand how natural selection might operate. Darwin and Wallace were unaware of the genetics work by Austrian monk Gregor Mendel, which was published in 1866, not long after publication of Darwin's book, On the Origin of Species. Mendel’s work was rediscovered in the early twentieth century at which time geneticists were rapidly coming to an understanding of the basics of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how gradual evolution could occur. But over the next few decades genetics and evolution were integrated in what became known as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today. In sum, the modern synthesis describes how evolutionary processes, such as natural selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory also connects this change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called macroevolution. Connection for AP® Courses Population genetics studies microevolution by measuring changes in a population’s allele frequencies over time. (Remember that we studied genotypes and allele frequencies when we explored inheritance patterns proposed by Mendel.) For example, scientists examining allele frequencies in a pesticide resistance gene in mosquitoes at Equatorial Guinea found that the frequency of one resistance allele was 6.3%, while a second resistance allele’s frequency was 74.6%, and the nonresistance allele’s frequency was 19.0%. These three frequencies add up to 100%. A population’s gene pool is the sum of all the alleles. If these frequencies do not change over time, the population is said to be in Hardy–Weinberg principle of |
equilibrium—a stable, non-evolving state. However, if a phenotype is favored by natural selection, allele frequencies can change. If this is the case, the population is evolving. Sometimes allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. This phenomenon is called genetic drift. An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, is called the founder effect. In Population Genetics, we will explore how natural selection, random drift, and founder effects can lead to significant changes in the genome of a population. [2] Hardy–Weinberg equilibrium reflects a state of constancy in a population’s gene pool. In other words, allele frequencies remain stable from generation to generation if certain conditions are met: no mutations, no gene flow, random mating, no genetic drift, and no selection. Because these conditions are rarely met, allele frequencies are typically changing, reflecting evolution. The Hardy–Weinberg principle is represented by the mathematical equation p2 + 2pq + q2 = 1, where p represents the frequency of the dominant allele and q represents the frequency of the recessive allele. Deviations from Hardy–Weinberg equilibrium allow us to measure microevolutionary shifts in a population when one or more of the Hardy–Weinberg parameters change. For example, if we go back to the study of the frequencies of alleles in a pesticide resistance gene, after an area was treated with pesticides for two years, the resistance alleles increased to 11.1% and 83.3%, respectively, while the non-resistance allele decreased to 5.6%. This indicates that microevolution was occurring. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. 2. Reddy, M. R., Godoy, A., Dion, K., Matias, A., Callender, K., Kiszewski, A. E., Slotman, M. A. (2013). Insecticide Resistance Allele Frequencies in Anopheles gambiae before and after Anti-Vector Interventions in Continental Equatorial Guinea |
. The American Journal of Tropical Medicine and Hygiene, 88(5), 897–907. doi:10.4269/ajtmh.12-0467 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 769 Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.A Essential Knowledge Science Practice Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Change in the genetic makeup of a population over time is evolution. 1.A.1 Natural selection is a major mechanism of evolution. 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. 1.A.1 Natural selection is a major mechanism of evolution. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. 1.A.1 Natural selection is a major mechanism of evolution. 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.3 The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.1] 770 Chapter 19 | The Evolution of Populations Evolution and Flu Vaccines Every fall, the media starts reporting on flu vaccinations and potential outbreaks. Scientists, health experts, and institutions determine recommendations for different parts of the population, predict optimal production and inoculation schedules, create vaccines, and set up clinics to provide inoculations. You may think of the annual flu shot as a lot of media hype, |
an important health protection, or just a briefly uncomfortable prick in your arm. But do you think of it in terms of evolution? The media hype of annual flu shots is scientifically grounded in our understanding of evolution. Each year, scientists across the globe strive to predict the flu strains that they anticipate being most widespread and harmful in the coming year. This knowledge is based in how flu strains have evolved over time and over the past few flu seasons. Scientists then work to create the most effective vaccine to combat those selected strains. Hundreds of millions of doses are produced in a short period in order to provide vaccinations to key populations at the optimal time. Because viruses, like the flu, evolve very quickly (especially in evolutionary time), this poses quite a challenge. Viruses mutate and replicate at a fast rate, so the vaccine developed to protect against last year’s flu strain may not provide the protection needed against the coming year’s strain. Evolution of these viruses means continued adaptions to ensure survival, including adaptations to survive previous vaccines. Population genetics is the study of what? a. How selective forces change the allele frequencies in a population over time. b. The genetic basis of genetic traits within individuals. c. Whether traits have a genetic basis. d. The degree of inbreeding in a population. Population Genetics Recall that a gene for a particular character may have several alleles, or variants, that code for different traits associated with that character. For example, in the ABO blood type system in humans, three alleles determine the particular blood-type protein on the surface of red blood cells. Each individual in a population of diploid organisms can only carry two alleles for a particular gene, but more than two may be present in the individuals that make up the population. Mendel followed alleles as they were inherited from parent to offspring. In the early twentieth century, biologists in a field of study known as population genetics began to study how selective forces change a population through changes in allele and genotypic frequencies. The allele frequency (or gene frequency) is the proportion of a specific allele within a population, relative to all other alleles of that gene that are present in the population. Until now we have discussed evolution as a change in the characteristics of a population of organisms, but behind that phenotypic change is genetic change. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Using the ABO blood type system as an example, the frequency of one of the |
alleles, IA, is the number of copies of that allele divided by all the copies of the ABO gene in the found a frequency of IA to be 26.1 percent. The IB and I0 alleles made up 13.4 population. For example, a study in Jordan percent and 60.5 percent of the alleles respectively, and all of the frequencies added up to 100 percent. A change in this frequency over time would constitute evolution in the population. [3] The allele frequency within a given population can change depending on environmental factors; therefore, certain alleles become more widespread than others during the process of natural selection. Natural selection can alter the population’s genetic makeup; for example, if a given allele confers a phenotype that allows an individual to better survive or have more offspring. Because many of those offspring will also carry the beneficial allele, and often the corresponding phenotype, they will have more offspring of their own that also carry the allele, thus, perpetuating the cycle. Over time, the allele will spread throughout the population. Some alleles will quickly become fixed in this way, meaning that every individual of the population will carry the allele, while detrimental mutations may be swiftly eliminated if derived from a dominant allele from the gene pool. The gene pool is the sum of all the alleles in a population. 3. Sahar S. Hanania, Dhia S. Hassawi, and Nidal M. Irshaid, “Allele Frequency and Molecular Genotypes of ABO Blood Group System in a Jordanian Population,” Journal of Medical Sciences 7 (2007): 51-58, doi:10.3923/jms.2007.51.58. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 771 Sometimes, allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. This phenomenon is called genetic drift. Natural selection and genetic drift usually occur simultaneously in populations and are not isolated events. It is hard to determine which process dominates because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence. An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, is called the founder effect. Natural selection, random drift, and founder effects can lead to significant changes in the genome of a population. Hardy–Weinberg |
Principle of Equilibrium In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg stated the principle of equilibrium to describe the genetic makeup of a population. The theory, which later became known as the Hardy–Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies are inherently stable—unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy–Weinberg principle assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population; while no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes. Working under this theory, population geneticists represent different alleles as different variables in their mathematical models. The variable p, for example, typically represents the frequency of the dominant allele, say Y for the trait of yellow in Mendel's peas. The variable q represents the frequency of the recessive allele, in this case y, that confers the color green. If these are the only two possible alleles for a given locus in the population, p + q = 1. In other words, all the p alleles and all the q alleles make up all of the alleles for that locus that are found in the population. But what ultimately interests most biologists is not the frequencies of different alleles, but the frequencies of the resulting genotypes, known as the population’s genetic structure, from which scientists can surmise the distribution of phenotypes. If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes. Since each individual carries two alleles per gene, if the allele frequencies (p and q) are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypes if two alleles are drawn at random from the gene pool. So in the above scenario, an individual pea plant could be pp (YY), and thus produce yellow peas; pq (Yy), also yellow; or qq (yy), and thus producing green peas (Figure 19.2). In other words, the frequency of pp individuals is simply p2; the frequency of pq individuals is 2pq; and the frequency of qq individuals is q2. And, again, if p and q are the |
only two possible alleles for a given trait in the population, these genotypes frequencies will sum to one: p2 + 2pq + q2 = 1. 772 Chapter 19 | The Evolution of Populations Figure 19.2 When populations are in the Hardy–Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined from the Hardy–Weinberg equation. If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play. In plants, violet flower color (V) is dominant over white (v). If p = 0.8 and q = 0.2 in a population of 500 plants, how many individuals would you expect to be homozygous dominant (VV), heterozygous (Vv), and homozygous recessive (vv)? How many plants would you expect to have violet flowers, and how many would have white flowers? a. homozygous dominant: 320 heterozygous: 160 homozygous recessive: 20 violet: 480 white: 20 b. homozygous dominant: 320 heterozygous: 80 homozygous recessive: 20 violet: 400 white: 20 c. homozygous dominant: 400 heterozygous: 0 homozygous recessive: 100 violet: 400 white: 100 d. homozygous dominant: 480 heterozygous: 0 homozygous recessive: 20 violet: 480 white: 20 In theory, if a population is at equilibrium—that is, there are no evolutionary forces acting upon it—generation after generation would have the same gene pool and genetic structure, and these equations would all hold true all of the time. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 773 Of course, even Hardy and Weinberg recognized that no natural population is immune to evolution. Populations in nature are constantly changing in genetic makeup due to drift, mutation, possibly migration, and selection. As a result, the only way to determine the exact distribution of phenotypes in a population is to go out and count them. But the Hardy–Weinberg principle gives scientists a mathematical baseline of a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play. If the frequencies of alleles or genotypes deviate from the value expected from the |
Hardy–Weinberg equation, then the population is evolving. Use this online calculator (http://openstaxcollege.org/l/hardy-weinberg) to determine the genetic structure of a population. What would violate the conditions of Hardy-Weinberg equilibrium? a. random mating b. mutations c. large population d. no natural selection Lab Investigation AP® Biology Investigative Labs: Investigation 2: Mathematical Modeling: Hardy–Weinberg. In this lab investigation, you apply the Hardy–Weinberg equation and create a spreadsheet to study changes in allele frequencies in a population and to examine possible causes for these changes. Inquiry-Based Approach, Think About It Imagine you are trying to determine if a population of flowers is undergoing microevolution. You suspect there is selection pressure on the color of the flower because bees seem to cluster around red flowers more often than blue flowers. In a separate experiment, you discover that blue flower color is dominant to red flower color. In a field, you count 600 blue flowers and 200 red flowers. Based on the H-W equation, what are the expected allele frequencies for flower color? Two years later, you revisit the same field and discover that out of 1,000 flowers, 650 are blue. Use the H–W equation to determine if the population of flowers is undergoing evolution. 774 Chapter 19 | The Evolution of Populations 19.2 | Population Genetics In this section, you will explore the following questions: • What are the different types of variation in a population? • Why can only heritable variation be acted upon by natural selection? • How can genetic drift, the bottleneck effect, and the founder effect influence allele frequencies in a population? • How can gene flow, mutation, nonrandom mating, and environmental variance affect allele frequencies in a population? Connection for AP® Courses Take a look at your classmates. Individuals of a population often display different phenotypes, or express different alleles of a particular gene. These differences are called polymorphisms. The distribution of phenotypes among individuals, known as population variation, is influenced by several factors, including the population’s genetic structure and the environment (Figure 19.3). Understanding the sources of phenotypic variation is important for determining how a population will evolve in response to different evolutionary pressures. Only those variations that are encoded in an individual’s genes can be passed to its offspring and be a target of natural selection. Figure 19.3 The distribution of phenotypes in this litter |
of kittens illustrates population variation. (credit: Pieter Lanser) As you learn in the chapter that discusses the evolution and origin of species, natural selection works by selecting for phenotypes—and the alleles that determine them—that confer beneficial traits or behaviors. Deleterious qualities are selected against. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and, thus, will pass on more of their genes to the next generation. Small and isolated populations are more susceptible to genetic drift. Natural events, such as wildfires or hurricanes, can magnify genetic drift when a large portion of the population is killed. Because a fire does not distinguish between the genotypes of various organisms, no particular genotype survives the fire better than another. Therefore, the genetic structure of the surviving population may be very different from the genetic structure of the original population. This is called the bottleneck effect. Another scenario in which populations might experience a strong influence of genetic drift occurs when some portion of the population leaves to start a new population in a new location or gets separated by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, a phenomenon called the founder effect. Both the bottleneck effect and the founder effect reduce genetic variation within a population—and genetic variation is the basis for natural selection. When individuals leave or join a population, they carry their alleles with them, resulting in changes in the population’s allele frequencies. Allele frequencies also can change due to mutation in DNA and when individuals do not randomly mate with others; when an individual selects a mate based on phenotype, the genotype is also selected. In summary, any of these conditions can result in deviations from the Hardy–Weinberg equilibrium—and lead to the microevolution of a population. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 775 foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.A Change in |
the genetic makeup of a population over time is evolution. Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective 1.A.1 Natural selection is a major mechanism of evolution. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. 1.A.2 Natural selection acts on phenotypic variations in populations. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 1.3 The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. 1.A.2 Natural selection acts on phenotypic variations in populations. 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.4 The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time. 1.A.3 Evolutionary change is also driven by random processes. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 1.8 The student is able to make predictions about the effects of genetic drift, migration, and artificial selection on the genetic makeup of a population. 1.A.3 Evolutionary change is also driven by random processes. 1.4 The student can use representatives and models to analyze situations or solve problems qualitatively and quantitatively. 2.1 The student can justify the selection of a mathematical routine to solve problems. 1.6 The student is able to use data from mathematical models based on the Hardy–Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. 1.A.3 Evolutionary change is also driven by random processes. 2.1 The student can justify the selection of a mathematical routine to solve problems. 1.7 The student is able to justify data from mathematical models based on the Hardy–Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. 776 Chapter 19 | The Evolution of Populations The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1. |
1][APLO 1.3][APLO 1.4][APLO 1.8][APLO 1.23][APLO 1.24][APLO 1.25][APLO 1.6][APLO 1.7][APLO 1.22] Genetic Variance Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child. Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck theorized that acquired traits could, in fact, be inherited; while this hypothesis has largely been unsupported, scientists have recently begun to realize that Lamarck was not completely wrong. Visit this site (http://openstaxcollege.org/l/epigenetic) to learn more. Explain naturalist Jean-Baptiste Lamarck’s theory on heritability. a. Lamarck theorized that individuals more fit to their environment would me more likely to survive, reproduce, and pass on their genes. b. Lamarck theorized that traits that parents acquired in their lifetime could be inherited by offspring in an attempt to improve. c. Lamarck theorized that traits in offspring were a blend of traits from the two parents. d. Lamarck theorized that inbreeding would lead to higher proportions of homozygous recessive genotypes, potentially conferring recessive diseases onto offspring. Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the |
phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression. Changes in allele frequencies that are identified in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 777 Genetic Drift The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure. Another way a population’ |
s allele and genotype frequencies can change is genetic drift (Figure 19.4), which is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting). 778 Chapter 19 | The Evolution of Populations Figure 19.4 Genetic drift occurs when the gene frequency of a population shifts by random chance (i.e. without a selective pressure). Over time, genetic drift can completely eliminate an allele from the population. For example, in the first generation here, the two alleles B and b occur with equal frequency, so p = q = 0.5. If only half the individuals reproduce, and by chance most of the reproducing alleles of are of B, then the second generation results in p = 0.7 and q = 0.3. In the second generation, only two individuals, both of whom are homozygote in B, reproduce. This leads to a loss of b from the third generation. As seen from the table, the frequency of the b allele is reduced as a percentage of the population due to genetic drift. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 779 Generation Individuals with genotype BB Individuals with genotype Bb Individuals with genotype bb 1 2 3 22 108 633 53 118 0 25 25 0 Table 19.1 Genotypic frequencies of rabbit populations undergoing genetic drift Explain why small populations are more vulnerable to the forces of genetic drift than large populations. a. The fewer individuals in a population, the more room there is for new individuals to migrate into it. b. The more individuals in a population, the more alleles are present in its gene pool. c. The fewer individuals in a population, the more likely it is to go extinct. d. The more individuals in a population, the larger and more stable is its gene pool. Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, |
all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure. Go to this site (http://openstaxcollege.org/l/genetic_drift) to watch an animation of random sampling and genetic drift in action. Describe an example of genetic drift. a. b. Immigration of new individuals can cause genetic drift. For example, if several white rabbits migrate into a population of mostly brown rabbits, the allele for white fur will increase within the population. Introduction of new alleles through mutation can cause genetic drift. For example, if there are two alleles for fur color in a rabbit population, and a mutation in one of them produces a third allele, the gene pool changes to incorporate the new allele. c. Chance events such as a natural disasters can cause genetic drift. For example, if the only white rabbits in a population get killed by a storm, the allele for white fur will diminish or disappear in the population. d. Differential survival and reproduction can cause genetic drift. For example, if all the white rabbits in a population get eaten by wolves because their white fur stands out and is more visible, the proportion of the allele for white fur in the population will decrease. Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. Known as the bottleneck effect, it results in a large portion of the genome suddenly being wiped out (Figure 19.5). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. 780 Chapter 19 | The Evolution of Populations Figure 19.5 A chance event or catastrophe can reduce the genetic variability within a population. Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of |
the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities. Watch this short video (http://openstaxcollege.org/l/founder_bottle) to learn more about the founder and bottleneck effects. Compare and contrast the bottleneck and founder effects. a. Both the bottleneck and founder effect are examples of gene flow. However, the bottleneck effect occurs after a cataclysmic event, whereas the founder effect occurs when mutations introduce new alleles into a population. b. Both the bottleneck and founder effect are examples of genetic drift. However, the bottleneck effect is a process in which a large portion of a genome is wiped out, whereas the founder effect occurs when members of a larger population migrate to establish their own population. c. Both the bottleneck and founder effect change the genetic structure of a population. However, the bottleneck effect reduces or eliminates alleles within a population, whereas the founder effect introduces or increases alleles. d. Both the bottleneck and founder effect change the genetic structure of a population. However, the bottleneck effect occurs when inbreeding depression kills off part of a population, whereas the founder effect relies on nonrandom mating. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 781 Testing the Bottleneck Effect Question: How do natural disasters affect the genetic structure of a population? Background: When much of a population is suddenly wiped out by an earthquake or hurricane, the individuals that survive the event are usually a random sampling of the original group. As a result, the genetic makeup of the population can change dramatically. This phenomenon is known as the bottleneck effect. Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each time this experiment is run, the results will vary. Test the hypothesis: Count out the original population using different colored beads. For example, red, blue, and yellow beads might represent red, blue, and yellow individuals. After recording the number of each individual in the original population, place them all in a bottle with a narrow neck that |
will only allow a few beads out at a time. Then, pour 1/3 of the bottle’s contents into a bowl. This represents the surviving individuals after a natural disaster kills a majority of the population. Count the number of the different colored beads in the bowl, and record it. Then, place all of the beads back in the bottle and repeat the experiment four more times. Analyze the data: Compare the five populations that resulted from the experiment. Do the populations all contain the same number of different colored beads, or do they vary? Remember, these populations all came from the same exact parent population. Form a conclusion: Most likely, the five resulting populations will differ quite dramatically. This is because natural disasters are not selective—they kill and spare individuals at random. Now think about how this might affect a real population. What happens when a hurricane hits the Mississippi Gulf Coast? How do the seabirds that live on the beach fare? Gene Flow Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure 19.6). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population, but it can also introduce new genetic variation to populations in different geological locations and habitats. Figure 19.6 Gene flow can occur when an individual travels from one geographic location to another. Mutation Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly 782 Chapter 19 | The Evolution of Populations eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in |
the genome. Some can have a dramatic effect on a gene and the resulting phenotype. Nonrandom Mating If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selected for by natural selection. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby. Environmental Variance Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment (Figure 19.7). For example, sun exposure is an environmental factor, as a person who spends more time in the sun will likely have darker skin than a person who spends most of their time indoors (assuming both people had similarly-colored skin to start with). Some major characteristics, such as sex, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range. Figure 19.7 The sex of the American alligator (Alligator mississippiensis) is determined by the temperature at which the eggs are incubated. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males. (credit: Steve Hillebrand, USFWS) Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient. Species of warmblooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of |
a mountain, known as an altitudinal cline. If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 783 Lab Investigation AP® Biology Investigative Labs: Investigation 1: Artificial Selection. Using Wisconsin Fast Plants, you explore evolution by conducting an artificial selection investigation to increase or decrease genetic variation in a population and then determine if extreme selection can change the expression of a quantitative trait. Inquiry-Based Approach, Think About It • Do you think genetic drift would happen more quickly on an island or on the mainland? Provide reasoning for your answer. • Consider the population of red and blue flowers you analyzed in Section 1 to determine if they were undergoing microevolution. Recall that you counted 600 blue flowers and 200 red flowers. • Imagine that you return four years after your initial visit, and the flowers at the site have been split into two different populations by a newly formed river, which isolates the two populations. In the population 1, you counted 125 blue flowers and 10 red flowers. In the population 2, you counted 450 blue flowers and 300 red flowers. Did genetic drift or natural selection likely cause these change in allele frequencies in population 1? What about population 2? Explain how you know for each population. 19.3 | Adaptive Evolution In this section, you will explore the following questions: • What are different ways in which natural selection can shape populations? • How can these different forces lead to different outcomes in terms of population variation? Connections for AP® Courses As we have learned, natural selection acts on the level of the individual, selecting those with a higher overall fitness (reproductive success) compared to the rest of the population. In other words, natural selection favors the most adaptive variation for a given environment. If the fit phenotypes are evolving in a stable environment, natural selection results in stabilizing selection, resulting in an overall decrease in the population’s variation. However, if environmental conditions change, directional selection shifts a population’s variance toward a new and more favorable phenotype. Diversifying selection results in increased variance by selecting for two or more distinct phenotypes. Sexual selection results when one sex has more reproductive success than the other; as a result, males and females experience |
different selective pressures, which often lead to distinct phenotypic differences, or sexual dimorphisms, between the two. For example, male birds often exhibit more colorful plumage than female birds of the same species. What is most important to recognize is that there is no perfect organism. Natural selection acts on existing variations in the population; it does not create anything from scratch. Although natural selection selects the fittest individuals, other forces of evolution, including genetic drift and gene flow, often introduce deleterious alleles to the population’s gene pool. Evolution has no purpose; it is simply the sum of various forces that influence the genetic and phenotypic variation of a population. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. 784 Chapter 19 | The Evolution of Populations Enduring Understanding 1.A Essential Knowledge Change in the genetic makeup of a population over time is evolution. 1.A.1 Natural selection is a major mechanism of evolution. Science Practice 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. Learning Objective Essential Knowledge 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. 1.A.2 Natural selection acts on phenotypic variations in populations. Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales. Learning Objective 1.5 The student is able to connect evolutionary changes in a population over time to a change in the environment. Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency—a process known as adaptive evolution. Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce (fecundity), but if that |
same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness. Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve. There are several ways selection can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate. Stabilizing Selection If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Figure 19.9). In a population of mice that live in the woods, for example, natural selection is likely to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most closely matched to that color will be most likely to survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them a bit lighter or a bit darker will stand out against the ground and be more likely to fall victim to predation. As a result of this selection, the population’s genetic variance will decrease. Directional Selection When the environment changes, populations will often undergo directional selection (Figure 19.9), which selects for phenotypes at one end of the spectrum of existing variation. A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. But as soot began spewing from factories, the trees became darkened, and the light-colored moths became easier for predatory birds to spot. Over time, the frequency of the melanic form of the |
moth increased because they had a higher survival rate in habitats affected by air pollution because their darker coloration blended with the sooty trees. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 785 Scientist can observe directional selection. Suppose populations of rabbits that eat flowers is introduced into an environment with flowering plants. Once the flowers are eaten, the plants cannot reproduce. Over time, the height of the flowers will shift higher so that the rabbits cannot reach them???. Figure 19.8 The introduction of small herbivores that eat flowers often results in directional selection for increased flower height. In science, sometimes things are believed to be true, and then new information comes to light that changes our understanding. The story of the peppered moth is an example: the facts behind the selection toward darker moths have recently been called into question. Read this article (http://openstaxcollege.org/l/peppered_moths) to learn more. What is fitness the measure of? a. the frequency of beneficial alleles b. the effect of chance on a population’s gene pool c. successful reproduction d. the abnormalities in a population 786 Chapter 19 | The Evolution of Populations Diversifying Selection Sometimes two or more distinct phenotypes can each have their advantages and be selected for by natural selection, while the intermediate phenotypes are, on average, less fit. Known as diversifying selection (Figure 19.9), this is seen in many populations of animals that have multiple male forms. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which can’t overtake the alpha males and are too big to sneak copulations, are selected against. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with |
either the grass or the sand, and would thus be more likely to be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 787 Figure 19.9 Different types of natural selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against. Before the Industrial revolution light-colored moths were able to blend in with the environment and better avoid predators. Since the Industrial Revolution, dark-colored moths are better camouflaged than light-colored moths. The number of dark-colored moths has increased to be the most common color. This is an example of what? a. directional selection b. c. stabilizing selection frequency-dependent selection d. diversifying selection Frequency-dependent Selection Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection). An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males (Figure 19.10) are the smallest, and look a bit like females, which allows them 788 Chapter 19 | The Evolution of Populations to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. That is, the big, strong orange males can fight off the blue males to mate with the blue’s pairbonded females, the blue males are successful at guarding their mates against yellow sneaker males, and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males. Figure 19.10 A yellow-throated side-bl |
otched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. (credit: “tinyfroglet”/Flickr) In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes—in one generation, orange might be predominant, and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored. Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes. Sexual Selection Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. Males are often larger, for example, and display many elaborate colors and adornments, like the peacock’s tail, while females tend to be smaller and duller in decoration. Such differences are known as sexual dimorphisms (Figure 19.11), which arise from the fact that in many populations, particularly animal populations, there is more variance in the reproductive success of the males than there is of the females. That is, some males—often the bigger, stronger, or more decorated males—get the vast majority of the total matings, while others receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to get those matings, resulting in the evolution of bigger body size and elaborate ornaments to get the females’ attention. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males. Sexual dimorphism varies widely among species, of course, and some species are even sex-role reversed. In such cases, females tend to have a greater variance in their reproductive success than males and are correspondingly selected for the bigger body size and elaborate traits usually characteristic of males. This |
OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 789 Figure 19.11 Sexual dimorphism is observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and in (c) wood ducks. (credit “spiders”: modification of work by “Sanba38”/Wikimedia Commons; credit “duck”: modification of work by Kevin Cole) The selection pressures on males and females to obtain matings is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help to maximize its reproductive success. Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual’s survival. Think, once again, about the peacock’s tail. While it is beautiful and the male with the largest, most colorful tail is more likely to win the female, it is not the most practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place. The speculation is that large tails carry risk, and only the best males survive that risk: the bigger the tail, the more fit the male. This idea is known as the handicap principle. The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring. 790 Chapter 19 | The Evolution of Populations In 1915, biologist Ronald Fisher proposed another model of sexual selection: the Fisherian runaway model (http://openstaxcollege.org/l/sexual_select), which suggests that selection of certain traits is a result of sexual preference. Explain the handicap principle. a. The peacock’s tail is an example of the handicap principle. Having a healthy |
, beautiful tail discourages predation, helping in survival. This means that those individuals are most likely to survive and produce offspring. b. The peacock’s tail is an example of the handicap principle. It appears that the tail makes the males more visible to predators and less able to escape, making it a disadvantage to the birds’ survival. However, traits cannot evolve in a population if they serve as a handicap to the individuals that express that trait. Therefore, the tail must actually be an advantage. c. The peacock’s tail is an example of the handicap principle. The tail makes the males more visible to predators and less able to escape, so the birds with the longest and most extravagant tails get eaten and do not reproduce. This causes the average tail length for males within the population to decrease over time due to natural selection. d. The peacock’s tail is an example of the handicap principle. The tail, which makes the males more visible to predators and less able to escape, is clearly a disadvantage to the birds’ survival. But because it is a disadvantage, only the most fit males should be able to survive with it. Thus, the tail serves as an honest signal of quality to the females of the population; therefore, the male will earn more matings and greater reproductive success. In both the handicap principle and the good genes hypothesis, the trait is said to be an honest signal of the males’ quality, thus giving females a way to find the fittest mates— males that will pass the best genes to their offspring. No Perfect Organism Natural selection is a driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments. But natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it does not create anything from scratch. Thus, it is limited by a population’s existing genetic variance and whatever new alleles arise through mutation and gene flow. Natural selection is also limited because it works at the level of individuals, not alleles, and some alleles are linked due to their physical proximity in the genome, making them more likely to be passed on together (linkage disequilibrium). Any given individual may carry some beneficial alleles and some unfavorable alleles. It is the net effect of these alleles, or the organism’s fitness, upon which natural selection can act. As a result, good alleles can be lost if |
they are carried by individuals that also have several overwhelmingly bad alleles; likewise, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit. Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency due to the fact that going from the less beneficial to the more beneficial trait would require going through a less beneficial phenotype. Think back to the mice that live at the beach. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of grass. The dark-colored mice may be, overall, more fit than the light-colored mice, and at first glance, one might expect the lightcolored mice be selected for a darker coloration. But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice—they cannot blend in with either the sand or the grass and are more likely to be eaten by predators. As a result, the light-colored mice would not be selected for a dark coloration because those individuals that began moving in that direction (began being selected for a darker coat) would be less fit than those that stayed light. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 791 Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and gene flow, often do the opposite: introducing deleterious alleles to the population’s gene pool. Evolution has no purpose—it is not changing a population into a preconceived ideal. It is simply the sum of the various forces described in this chapter and how they influence the genetic and phenotypic variance of a population. Think About It In recent years, factories have been cleaner, and less soot is released into the environment. What impact do you think this has had on the distribution of moth color in the population? 792 Chapter 19 | The Evolution of Populations KEY TERMS adaptive evolution increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection allele frequency (also, gene frequency) rate at which a specific allele appears within a population assortative mating when individuals tend to mate with those who are phen |
otypically similar to themselves bottleneck effect magnification of genetic drift as a result of natural events or catastrophes cline gradual geographic variation across an ecological gradient directional selection selection that favors phenotypes at one end of the spectrum of existing variation diversifying selection selection that favors two or more distinct phenotypes evolutionary fitness (also, Darwinian fitness) individual’s ability to survive and reproduce fitness measure of successful reproduction, the passing on alleles to the next generation founder effect population event that initiates an allele frequency change in part of the population, which is not typical of the original frequency-dependent selection selection that favors phenotypes that are either common (positive frequency- dependent selection) or rare (negative frequency-dependent selection) gene flow flow of alleles in and out of a population due to the migration of individuals or gametes gene pool all of the alleles carried by all of the individuals in the population genetic drift effect of chance on a population’s gene pool genetic structure distribution of the different possible genotypes in a population genetic variance diversity of alleles and genotypes in a population genotype frequency the proportion of a specific genotype in a population relative to all other genotypes for those genes that are present in the population geographical variation differences in the phenotypic variation between populations that are separated geographically good genes hypothesis theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease handicap principle theory of sexual selection that argues only the fittest individuals can afford costly traits Hardy–Weinberg principle of equilibrium a stable, non-evolving state of a population in which allelic frequencies are stable over time heritability fraction of population variation that can be attributed to its genetic variance honest signal trait that gives a truthful impression of an individual’s fitness inbreeding mating of closely related individuals inbreeding depression increase in abnormalities and disease in inbreeding populations macroevolution broader scale evolutionary changes seen over paleontological time microevolution changes in a population’s genetic structure modern synthesis overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 793 nonrandom mating changes in a population’s gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others polymorphisms variations in phenotype within individuals of a population population genetics study of how selective forces change the |
allele frequencies in a population over time population variation distribution of phenotypes in a population relative fitness individual’s ability to survive and reproduce relative to the rest of the population selective pressure environmental factor that causes one phenotype to be better than another sexual dimorphism phenotypic difference between the males and females of a population stabilizing selection selection that favors average phenotypes CHAPTER SUMMARY 19.1 Population Evolution The modern synthesis of evolutionary theory grew out of the cohesion of Darwin’s, Wallace’s, and Mendel’s thoughts on evolution and heredity, along with the more modern study of population genetics. It describes the evolution of populations and species, from small-scale changes among individuals to large-scale changes over paleontological time periods. To understand how organisms evolve, scientists can track populations’ allele frequencies over time. If they differ from generation to generation, scientists can conclude that the population is not in Hardy–Weinberg equilibrium, and is thus evolving. 19.2 Population Genetics Both genetic and environmental factors can cause phenotypic variation in a population. Different alleles can confer different phenotypes, and different environments can also cause individuals to look or act differently. Only those differences encoded in an individual’s genes, however, can be passed to its offspring and, thus, be a target of natural selection. Natural selection works by selecting for alleles that confer beneficial traits or behaviors, while selecting against those for deleterious qualities. Genetic drift stems from the chance occurrence that some individuals in the germ line have more offspring than others. When individuals leave or join the population, allele frequencies can change as a result of gene flow. Mutations to an individual’s DNA may introduce new variation into a population. Allele frequencies can also be altered when individuals do not randomly mate with others in the group. 19.3 Adaptive Evolution Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype, as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by selecting for two or more |
distinct phenotypes. Other types of selection include frequency-dependent selection, in which individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) are selected for. Finally, sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two. REVIEW QUESTIONS 1. What is the ultimate source of all variation in and among populations? 794 Chapter 19 | The Evolution of Populations a. genetic mutations that result in viable offspring a. Natural selection leads to changes in organisms b. natural selection c. diverse habitats d. factors in the environment that may affect development 2. When male lions reach sexual maturity, they are thrown out of their group, or pride, and must live on their own or with other males until they can take over their own pride. This can alter the allele frequencies of the population through which of the following mechanisms? over time b. The strong arms of a gorilla are the result of its parents constantly climbing, lifting and fighting. c. Lack of resources led to the death of three of four fox cubs. d. The founder effect is when a few individuals in a population are separated from the original population. 6. What is population variance influenced by? a. natural selection b. gene flow c. random mating d. genetic drift 3. Which of the following populations has violated the conditions of Hardy-Weinberg Equilibrium? a. an infinitely large population b. a population in which the allele frequencies do not change over time c. a population in which the Hardy-Weinberg equation is equal to 1 d. a population undergoing natural selection 4. What is the difference between micro and macroevolution? a. Microevolution describes the evolution of small organisms, such as insects, while macroevolution describes the evolution of large organisms, like people and elephants. b. Microevolution describes the evolution of microscopic entities, such as molecules and proteins, while macroevolution describes the evolution of whole organisms. c. Microevolution describes the evolution of organisms in populations, while macroevolution describes the evolution of species over long periods of time. d. Microevolution describes the evolution of organisms over their lifetimes, while macroevolution describes the evolution of organisms over multiple generations. 5. Which of the following would be supported by Lamarck? |
a. genetic structure b. environment c. diet composition d. All of the above 7. What is genetic variance? a. b. c. d. the change in a population’s genetic structure the effect of chance on a population’s gene pool the diversity of alleles and genotypes within a population the magnification of genetic drift as a result of natural events or catastrophes 8. When closely related individuals mate with each other, or inbreed, the offspring are often not as fit as the offspring of two unrelated individuals. Why? a. Inbreeding causes normally silent alleles to be expressed. b. The DNA of close relatives reacts negatively in the offspring. c. Inbreeding can bring together rare, deleterious mutations that lead to harmful phenotypes d. Close relatives are genetically incompatible. 9. What could cause genetic drift to occur within a population? a. accidental deaths b. predators c. disease d. lack of gene flow 10. What is the evolutionary mechanism that alters allele frequencies by chance called? a. genetic drift b. natural selection c. inbreeding d. migration 11. What is assortative mating? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 795 a. when individuals mate with those who are similar to themselves 14. What types of phenotypes does negative frequencydependent selection favor? b. when individuals mate with those who are a. advantageous dissimilar to themselves c. when individuals mate with those who are most fit in the population d. when individuals mate with those who are least fit in the population 12. What is an example of a cline? a. a random fluctuation in a species gene frequencies b. a mutation that spreads across the ecological range of a species c. the females of a species preferring males that are orange in coloration instead of white d. a species having greater cold tolerance in the colder parts of its range than in the warmer parts of its range 13. Which type of selection results in greater genetic variance in a population? a. stabilizing selection b. directional selection c. diversifying selection d. positive frequency-dependent selection CRITICAL THINKING QUESTIONS b. rare c. common d. disadvantageous 15. The good genes hypothesis is a theory that explains what? a. why more fit individuals are more likely to have more offspring b. why alleles that confer beneficial traits or behaviors are selected for |
by natural selection c. why some deleterious mutations are maintained in the population d. why individuals of one sex develop impressive ornament traits 16. Which of the following describes when males and females of a population look or act differently/ a. sexual selection b. diversifying selection c. sexual dimorphism d. a cline 17. Describe natural selection and give an example of natural selection at work in a population. count 600 blue flowers and 200 red flowers. What would you expect the genetic structure of the flowers to be? a. The process in which genes flow from one population to another. The beak size of Darwin’s finches changing as the availability of differentsized seeds changes. b. The process in which genes flow from one population to another. The Founder Effect occurring among humans immigrating to a new country. c. The process in which better-adapted organisms are able to survive and reproduce; The beak size of Darwin’s finches changing as the availability of different-sized seeds changes. d. The process in which better-adapted organisms are able to survive and reproduce; The Founder Effect occurring among humans immigrating to a new country. a. You would expect 300 homozygous dominant blue flowers, 300 heterozygous blue flowers, and 200 homozygous recessive red flowers. b. You would expect 200 homozygous dominant blue flowers, 400 heterozygous blue flowers, and 200 homozygous recessive red flowers. c. You would expect 100 homozygous dominant red flowers, 100 heterozygous red flowers, and 600 homozygous recessive blue flowers. d. You would expect 14 homozygous dominant red flowers, 186 heterozygous blue flowers, and 600 homozygous recessive blue flowers. 19. What must occur in order for a new trait to appear in a population and then reach a steady, high frequency within that population? 18. Imagine you are trying to test whether a population of flowers is undergoing evolution. You suspect there is selection pressure on the color of the flower: bees seem to cluster around the red flowers more often than the blue flowers. In a separate experiment, you discover that blue flower color is dominant to red flower color. In a field, you 796 Chapter 19 | The Evolution of Populations a. New traits appear through gene mutations or a. A breeder would not allow close relatives to mate because inbreeding increases the likelihood of fatal mutations in offspring. b. A breeder would not allow close relatives |
to mate because inbreeding prevents gene flow which can bring new, successful genes into the population. c. A breeder would not allow close relatives to mate because inbreeding causes diversifying selection, which dilutes the breeder’s desired genes in the population. d. A breeder would not allow close relatives to mate because inbreeding can bring together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. 22. Explain the founder effect and identify the best example. through genetic drift. In order to reach a steady, high frequency in the population, there must be many mutagens, such as UV radiation, in the environment to produce many mutations. b. New traits appear through gene mutations or through genetic drift. In order to reach a steady, high frequency in the population, there must be a consistent source of immigrant individuals with the allele. c. New traits appear through gene mutations or through evolution. In order to reach a steady, high frequency in the population, the allele must code for a favorable adaptation. d. New traits appear through gene mutations or through gene flow. In order to reach a steady, high frequency in the population, the trait associated with the gene must be favored by either natural or sexual selection. 20. Define and identify an example of population variation. a. Population variation is a description of the diversity of different forms of life. An example of population variation would be the different forms and functions of prokaryotes versus eukaryotes. b. Population variation is the geographic distribution of different phenotypes in a population. An example of population variation would be the fact that warm-blooded mammals that live near the poles tend to be larger than their southern counterparts to conserve heat. c. Population variation is the distribution of phenotypes in a population. An example of population variation would be the many different fur colors and patterns found in domestic dogs. d. Population variation is the distribution of genotypes in a population. An example of population variation would be Mendel’s pea plants that were homozygous dominant, heterozygous and homozygous recessive for various traits. 21. People who breed domesticated animals try to avoid inbreeding even though most domesticated animals are indiscriminate. Evaluate why this is a good practice. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 797 a. The founder effect is an event that isol |
seal due to the loss of some alleles. c. The founder effect is when only a few males within a population are selected by females to reproduce, generating an allele frequency which is different from the original population. An example of the founder effect is the reproductive pattern of mountain gorillas. Mountain gorillas tend to have a single dominant male, the silverback, who gets the vast majority of the matings in the population. This leads to the next generation expressing mainly genes from the silverback and very few genes from the other males, changing the genetic structure of the population. d. The founder effect occurs when the selective pressure on a trait varies depending on the alleles expressed within the population, generating varying allele frequencies based on the genetic makeup of the original population. An example of the founder effect is the cyclical dominance of three throat-color patterns in sideblotched lizards. 23. Explain what a cline is and identify an example. a. A cline is a type of geographic variation that is seen in populations of a given species that vary gradually across an ecological gradient. For example, endothermic animals tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. b. A cline is a change in ecological conditions over a geographic distance. For example, a latitudinal cline is the decrease in temperature towards the Earth’s poles, and an altitudinal cline is the decrease in temperature with increase in altitude. c. A cline is the specific set of traits in a population of a given species that have been influenced by the local environment. For example, a population of warm-blooded animals that lived in a cooler climate closer to the North Pole would have larger bodies, allowing them to better conserve heat. d. A cline is the specific set of ecological conditions in a geographic region. For example, towards the North Pole it is cold and there is little precipitation. This will influence the traits of the organisms that live there. 24. The table below shows data for a small population of mice. The mice are either brown or white. Based on the data, is the population experiencing genetic drift? Explain. Generation Brown mice Black mice 14 20 24 21 19 24 1 2 3 4 5 6 Table 19.2 32 26 22 28 30 29 25. The large alpha male elephant seal is constantly fending off the advances of medium sized males. Small males are then able to sneak copulation with females and successfully pass on their genes |
. What is this an example of? Explain. 798 Chapter 19 | The Evolution of Populations a. This is an example of sexual selection. The females are selecting the small males over the large male. b. This is an example of genetic drift. Because there are so many medium-sized males to compete with the large alpha male, the small males are able to mate and cause the gene pool to shift towards smaller individuals. c. This is an example of positive frequency- dependent selection, which is selection that favors phenotypes that are either common or rare. The sneaky males (rare) are favored in this case. d. This is an eample of directional selection. Because only the smallest males are mating, the next generation will have a higher proportion of alleles for small size, making the seals smaller over time. 26. Explain why there is no perfect organism despite natural selection. a. Because natural selection works on a geographic level. b. Because natural selection works in a random manner like mutations. c. Because of limitations due to a population’s existing variation in genes. d. Because natural selection is limited to sexual dimorphism. 27. A new predator invades the habitat of a population of TEST PREP FOR AP® COURSES 29. A scientist is studying the genetics of a population of plants that she suspects is undergoing natural selection. After examining samples of the population’s DNA over several years, she finds the following data: Year Allele A Frequency Allele B Frequency 1 2 3 4 5 6 0.80 0.72 0.66 0.52 0.45 0.39 0.2 0.28 0.34 0.48 0.55 0.61 Does this provide evidence of natural selection in this population? Why or why not? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 field mice. Individuals with larger body size are easier for the predator to capture then individuals with smaller body size. Draw a histogram of body sizes with two plot lines, one showing the former population and another showing the new population that indicates how this population will likely evolve. On your histogram, also indicate what type of natural selection is occurring here. 28. Quinine is an antimalarial drug that is used to treat malaria in the Western Hemisphere. Scientists have noticed that this drug has become less effective over time. Based on the data below, what type of selection is being |
exerted on the malaria population? Figure 19.12 a. No, because the genotype frequencies, not allele frequencies, have to change for evolution to occur. b. No, because the allele frequencies are changing randomly, suggesting that genetic drift is occurring, not natural selection. c. Yes, because it shows that the previously favorable or neutral allele A is now being selected against in favor of allele B. d. Yes, because it is showing that the frequency of both alleles are changing over time. 30. A scientist is studying two large populations of deer that are centralized in nearby forests. She takes blood samples from all of the deer in each population and records in how many individuals she finds allele A. She then computes the frequency of the alleles in each population. The frequencies observed over five years are shown in the tables below. Chapter 19 | The Evolution of Populations 799 Year 2010 2011 2012 2013 2014 2015 Mowed field flowering date Unmowed field flowering date 7/29 7/20 7/13 7/8 7/1 6/29 7/28 7/26 8/1 7/29 8/2 7/26 a. The grass population is adapting to the mowing, so it can flower for longer before being mowed. b. Mowing stabilizes the flowering time, which follows a steady trend in the mowed field but not in the unmowed field. c. The mowing is preventing the grass from reproducing, causing the mowed field to adapt by flowering earlier. d. The grass typically flowers earlier and earlier every year as it becomes older with each passing year. 32. A scientist observed two populations of insects for 10 years. They took data on the length, in mm, of the insect’s mouthparts. Their data is shown in the graphs below. How is this population evolving and what agent of evolution is most likely at work? Which forms of evolution are most likely occurring in populations A and B? Explain your answer. a. b. c. d. In population A, genetic drift is likely occurring, causing allele A to become more prevalent than allele B. In population B, mutation apparently occurred, introducing allele A to population B. Allele A also appears to be increasing due to genetic drift in population B. In population A, natural selection is likely occurring, with allele A being favored over allele B. In population B, gene flow apparently occurred, allowing allele A to become established in population B. Allele A also appears to be favored |
by selection in population B. In population A, gene flow apparently occurred, allowing allele B to become established in population A. Allele A also appears to be favored by selection in population A. In population B genetic drift is likely occurring, causing allele A to become more prevalent than allele B. In population A, mutation apparently occurred, introducing allele B to population A. Allele A also appears to be increasing due to genetic drift in population A. In population B natural selection is likely occurring, with allele A being favored over allele B. 31. A land manager mows a section of annual grass. Over the years, he recorded the date of flowering from the mown field as well as a similar grass field that was not mown. What is the most likely explanation for this trend? 800 Chapter 19 | The Evolution of Populations a. non-random mating; both alleles are favored b. gene flow; allele A is favored c. genetic drift; both alleles are neutral d. natural selection; allele a is not favored 36. The graph below shows the change in gene frequency of the two alleles: A and B. These alleles are located on separate genes that do not influence each other in any way. The population being studied has no emigration or immigration. Which type of evolution is likely occurring here, if at all? Explain how you know. a. inbreeding, because the gene distributions are becoming less similar among the population b. genetic drift, as the distribution of traits has become more random c. gene flow, as the population has likely gained new mouthpart traits through immigration d. natural selection, as insects that have mid-sized mouthparts are being favored 33. A pond is stocked with 250 fish, all of the same species. At that time, the researchers tested the fish to determine if they were genetically predisposed to a certain disease. The gene tested has two alleles, A and a. They found that 58 of the fish possessed allele A, while the rest of the fish possessed allele a. They plan to reassess the fish 5 years later. A computer model predicts that the population will likely increase to 850 fish and have 403 heterozygote (Aa) individuals. Will the future population have evolved? State how you know. a. Evolution has not occurred, because the frequency of the heterozygotes is different 5 years later compared with the original population. b. Evolution has not occurred, because the frequency of the heterozygotes is different 5 years later as in |
the original population. c. Evolution has not occurred, because the frequency of the heterozygotes is the same 5 years later as in the original population. d. Evolution has occurred, because the frequency of the heterozygotes is different 5 years later compared with the original population. 34. Heterozygote advantage is a condition in which heterozygotes are favored by natural selection. How would the value of 2pq likely change if the population was undergoing heterozygote advantage? a. b. c. d. It would remain in equilibrium because the value of p and q would remain the same. It would remain in equilibrium because the value of 2pq would remain the same. It would not remain in equilibrium because the value of 2pq would likely increase. It would not remain in equilibrium because the value of 2pq would likely decrease. 35. The graph below shows the change in gene frequency of the two alleles of a gene: A and a. The population being studies has no emigration or immigration. Which type of evolution is likely occurring here and is the allele selected for, neutral, or selected against by natural selection? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 19 | The Evolution of Populations 801 a. Both AA and aa will drop in frequency by the same amount. b. Both AA and aa will drop, but aa will drop more. c. AA will increase in frequency and aa will drop in frequency. d. aa will increase in frequency and AA will drop in frequency. 38. The diagram below shows the frequency of alleles on two species of wind-pollinated plants, as well as the prevailing wind direction. These frequencies have been fairly stable for around 10 years. However, climate change has created a new prevailing wind direction, as shown in the diagram. How will the two populations likely evolve in the future? a. no multiple choice available b. no multiple choice available 37. The graph below shows the current frequencies of two genotypes of the same gene: AA and aa. What would most likely happen to the frequencies of A and a if heterozygous individuals were favored by natural selection? a. natural selection will cause the frequency of B to increase in population 1 b. gene flow will cause the frequencies of A and B to drop in population 3 c. genetic drift will cause the frequencies of A and C to increase in population 1 and 2 |
d. inbreeding will reduce the frequency of allele B in population 2 and 3 39. The diagram below shows two populations of organisms that have been long-separated by a river which prevents interbreeding. The two populations differ in coloration, as shown in the diagram. Recent human 802 Chapter 19 | The Evolution of Populations activity has caused the river to dry, however, resulting in the two populations shown in the lower diagram. What is the most likely explanation for this change? a. an increase in gene flow between the two populations b. a decrease in gene flow between the two populations c. an increase in non-random mating between the two populations d. a decrease in non-random mating between the two populations 40. Antibiotics are medicines that are designed to kill disease-causing organisms, or pathogens. However, some pathogens evolve antibiotic resistance, where they gain traits that allow them to survive in the presence of antibiotics. The ability of bacteria to adapt to antibiotics so quickly has created a huge concern over whether antibiotics are being overused. What form of evolution is antibiotic resistance an example of, and why? a. Gene flow because the bacteria are passing on the resistant trait within their populations. b. Natural selection, because the bacteria is adapting to a new environmental condition - the presence of the antibiotic. c. Genetic drift because medical workers cannot follow the randomly-fluctuating gene frequencies of bacterial populations d. Mutation, because each bacteria must mutate to an antibody resistant form in order to survive. SCIENCE PRACTICE CHALLENGE QUESTIONS 41. Consider a polymorphic gene with three alleles: A, B, and C. A. If the frequencies of the alleles A and B are 0.2 and 0.3, the frequency of allele C is closest to ___. a. 0.25 b. 0.5 c. 0.2 d. 0.3 Consider a gene with only two alleles: dominant A and recessive a. In a population of 1,000 organisms, the fraction expressing the homozygous recessive phenotype is 0.37. B. The calculated allele frequencies p and q have values that are closest to ___. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 a. 0.69 and 0.31 b. 0.31 and 0.69 c. 0.37 and 0.63 d. 0.63 and 0.37 Genotype C1 |
C1 C2 C2 C3 C3 C1 C2 C1 C3 C2 C3 Total Observed 120 230 112 175 198 165 1,000 Table 19.3 C. The calculated number of individuals in this population that are heterozygotes is closest to ___. Chapter 19 | The Evolution of Populations 803 a. 240 b. 230 c. 430 d. 476 Mountain pine beetles (Dendroctonus ponderosae) were collected from a one-acre tract of lodge pole pine trees (Pinus contorta) in a region of British Columbia where the forests are under temperature stress. The beetles were crushed, and a cellulase enzyme was extracted. Three polymorphs of the enzyme were observed when separated by gel electrophoresis. The three proteins observed correspond to alleles labeled C1, C2, and C3. The numbers of beetles with each allele are shown in the following table. D. The calculated allelic frequencies pC1, pC2, and pC3 are closest to ___. a. pC1 = 0.57 pC2 = 0.57 pC3 = 0.59 b. pC1 = 0.29 pC2 = 0.29 pC3 = 0.42 c. pC1 = 0.61 pC2 = 0.80 pC3 = 0.59 d. pC1 = 0.31 pC2 = 0.40 pC3 = 0.29 E. In order to investigate the presence of selection at the cellulase locus due to changing temperature, a biologist should: a. calculate the values of the sums pC1 + pC2 + pC3 and (pC1 + pC2 + pC3)2. If these numbers are not equal to 1, the gene is not in Hardy-Weinberg equilibrium, and the gene is evolving. b. c. d. return next year and repeat this examination of the enzyme, calculating frequencies of each allele each year. Then calculate the values of the sums pC1 + pC2 + pC3 and (pC1 + pC2 + pC3)2. If these numbers are not the same each year, the gene is not in Hardy-Weinberg equilibrium, and the gene is evolving. return each year for several years and repeat this examination of the enzyme, calculating frequencies of each allele each year. If the allele frequencies are changing, |
the gene is not in Hardy-Weinberg equilibrium, and temperature is exerting a selection pressure. return each year for several years and repeat this examination of the enzyme, calculating frequencies of each allele each year. If the allele frequencies are changing, the gene is not in Hardy-Weinberg equilibrium. Analysis of the dependence of allele frequencies on temperature could indicate selection. 42. Calamus finmarchicus is the dominant copepod in the Gulf of Maine. The polymorphic aminopeptidase locus, Lap-1, has been shown to be useful for the genetic differentiation of populations of this organism. By examining the population dynamics of copepods, the dynamics of the fin fish on which they feed can be predicted. The aerial photograph shows a landmass separating two coastal estuarine habits, the mud flats of Egypt Bay and the Mount Desert Narrows. For the past 40 years, transport between the two habits has been hindered by a dam over the Carrying Place Inlet. However, small volumes of water occasionally crest the dam. Figure 19.13 To evaluate the geographic isolation of invertebrate populations in these two habitats, copepods are sampled at the points labeled 1 and 2 on the photograph. These points lie at either ends of the Carrying Place Inlet. Enzymes encoded by three alleles, labeled A, B, and C, were determined by gel electrophoresis of equal numbers of the organisms collected at the two sites. Numbers of each genotype are given in the following table: Site 1 2 AA 82 96 AB 114 108 AC 102 BB 74 92 54 BC 98 110 CC 30 40 Total 500 500 Table 19.4 A. Calculate the frequencies, f, of each allele and complete the following table: 804 Chapter 19 | The Evolution of Populations Site f(A) f(B) f(C) summarize their results, comparing the natural logarithm of the number of species in each lineage. 1 2 Table 19.5 B. Using a χ2 test, evaluate these data to determine if the aminopeptidase gene in these two populations is evolving. State your conclusion as claims supported by evidence at both the 95% and 99% confidence levels. The formula for the χ2 test is provided on the AP Biology Exam. χ 2 = ∑ (o − e)2 e This table of critical p values is also provided on the AP Biology Exam. Degrees of Freedom.05 3.84 |
5.99 7.82 9.49 11.07 12.59 14.07 15.51 0.01 6.64 9.32 11.34 13.28 15.09 16.81 18.48 20.09 Table 19.6 C. Based on these data, predict, with justification, changes over time in the aminopeptidase enzyme for these populations. D. The B form of this aminopeptidase is slightly more efficient at extracting nutritional leucine from a protein than the A and C forms but slightly less efficient at extracting valine and serine. Describe an investigation of the two habitats that could suggest a causal relationship between changes in allele frequency and characteristics of the environment. E. Single-nucleotide mutations are neutral when they encode changes in proteins that result in no significant differential selection. If differences in environmental factors between sites 1 and 2 are not observed, predict what other factors could result in departures from HardyWeinberg equilibrium for aminopeptidase. 43. Bioluminescence is an example of convergent evolution; 30 distinct lineages have acquired this characteristic, and all involve some form of a class of molecules called luciferins. Sexual selection pressures are strong for light-emitting organisms. Ellis and Oakley (Curr Biol, 2016) examined the number of species that lack luminosity in groups of closest evolutionary relation (sister linear) with those species that are luminous. Similarly, scientists made the same comparison between groups that use luminosity for concealment (counterillumination) and their sister lineages. The graphs This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Figure 19.14 Based on the data shown in the graphs, describe a model that can account for the increased speciation of bioluminescent lineages, including the mechanism of speciation. 44. Chapter 19 | The Evolution of Populations 805 Figure 19.15 A biologist is using a simulation to model populations of African hornbills (Bycanistes spp. and Ceratogymna spp.), a keystone species of the savanna. Populations of the birds are declining due to habitat loss. The hornbill’s diet consists primarily of termites and fruit. A critical component of termite digestion is chitin deacetylase, an enzyme whose mutation rate is a model parameter. The other model parameter is population |
size, N. In the results of the simulation study shown above, there is no selection, and the mutation rate is fixed. Although both population size and mutation rate are fixed, randomness results in the five different outcomes shown in each graph above. A. Select the graph displaying the results that are closer to Hardy-Weinberg equilibrium. Justify the selection of the graph. B. Based on these simulations, predict the future heterozygosity, 2pq, of the smaller populations, as shown in graph A. C. Justify the use of a simulation study with no selection under environmental conditions in which the availability of both termites and fruit is high. D. If a change in the environment occurs suddenly, such as an increase in average temperature, where fruit production declines, analyze the effect of the change on allele frequency in the large and small populations. 806 Chapter 19 | The Evolution of Populations This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 807 20 | PHYLOGENIES AND THE HISTORY OF LIFE Figure 20.1 The life of a bee is very different from the life of a flower, but the two organisms are related. Both are members of the domain Eukarya and have cells containing many similar organelles, genes, and proteins. (credit: modification of work by John Beetham) Chapter Outline 20.1: Organizing Life on Earth 20.2: Determining Evolutionary Relationships 20.3: Perspectives on the Phylogenetic Tree Introduction This bee and Echinacea flower (Figure 20.1) could not look more different, yet they are related, as are all living organisms on Earth. By following pathways of similarities and changes—both visible and genetic—scientists seek to map the evolutionary past of how life developed from single-celled organisms to the tremendous variety of creatures that have germinated, crawled, floated, swam, flown, and walked on this planet. New species are discovered with frequent regularity, but it’s not too common to discover a new large mammal. However, that’s what scientists did in Australia when they named a new species of cetacean the Australian humpback dolphin, Souse sahulensis. The dolphin had originally been classified as another closely related species, but a closer look at its coloration, skeletal structure, habitat |
, and DNA determined that it was in fact a separate species. For more information, read the research article (http://openstaxcollege.org/l/32dolphin) yourself. 808 Chapter 20 | Phylogenies and the History of Life 20.1 | Organizing Life on Earth In this section, you will explore the following questions: • Why do scientists need a comprehensive classification system to study living organisms? • What are the different levels of the taxonomic classification system? • How are systematics and taxonomy related to phylogeny? • What are the components and purpose of a phylogenetic tree? Connection for AP® Courses In prior chapters we explored how all organisms on Earth, extant and extinct, evolved from common ancestry. Supporting this claim are core features and processes, such as a common genetic code and metabolic pathways, which evolved billions of years ago and are widely distributed among organisms living today. The evolutionary history and relationship of an organism or a group of organisms is called phylogeny. Scientists often construct phylogenetic trees based on evidence drawn from multiple disciplines to illustrate evolutionary pathways and connections among organisms. Scientists historically organized Earth’s millions of species into a hierarchical taxonomic classification system from the most inclusive category to the most specific: domain, kingdom, phylum, class, order, family, genus, and species. The traditional five-kingdom system that you might have studied in middle school was expanded (and reorganized) to include three domains: Bacteria, Archaea, and Eukarya, with prokaryotes divided between Bacteria or Archaea depending on their molecular genetic machinery, and protists, fungi, plants, and animals grouped in Eukarya. Today, however, phylogenetic trees provide more specific information about evolutionary history and relationships among organisms. (For the purpose of AP®, you do not have to memorize the taxonomic levels. However, it is important to reiterate that taxonomy is a tool to organize the millions of organisms on Earth, similar to how items in a grocery store or mall shop are organized into different departments. Like new products, organisms are often shifted among their taxonomic groups!) Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more |
of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 3.1 The student can pose scientific questions. 1.14 The student is able to pose scientific questions that correctly identify essential properties of shared, core life processes that provide insight into the history of life on Earth. Essential Knowledge 1.B.1 Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 809 Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 6.1 The student can justify claims with evidence. 1.16 The student is able to justify the scientific claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Essential Knowledge 1.B.2 Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Science Practice Learning Objective 3.1 The student can pose scientific questions. 1.17 The student is able to pose scientific questions about a group of organisms whose relatedness is described by a phylogenetic tree or cladogram. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.20][APLO 1.26] Phylogenetic Trees Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to |
be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms (Figure 20.2). Unlike a taxonomic classification diagram, a phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains— Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared with other organisms. Unrooted trees don’t show a common ancestor but do show relationships among species. Figure 20.2 Both of these phylogenetic trees shows the relationship of the three domains of life—Bacteria, Archaea, and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor while the (b) unrooted tree does not. (credit a: modification of work by Eric Gaba) In a rooted tree, the branching indicates evolutionary relationships (Figure 20.3). The point where a split occurs, called a branch point, represents where a single lineage evolved into a distinct new one. A lineage that evolved early from the root and remains unbranched is called basal taxon. When two lineages stem from the same branch point, they are called sister taxa. A branch with more than two lineages is called a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. It is important to note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split apart at a specific branch point, but neither taxa gave rise to the other. 810 Chapter 20 | Phylogenies and the History of Life Figure 20.3 The root of a phylogenetic tree indicates that an ancestral lineage gave rise to all organisms on the tree. A branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is |
a basal taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy. The diagrams above can serve as a pathway to understanding evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover that species' ancestors, as well as where lineages share a common ancestry. In addition, the tree can be used to study entire groups of organisms. Another point to mention on phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point was rotated and the taxon order changed, this would not alter the information because the evolution of each taxon from the branch point was independent of the other. Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Information is used to organize and classify organisms based on evolutionary relationships in a scientific field called systematics. Data may be collected from fossils, from studying the structure of body parts or molecules used by an organism, and by DNA analysis. By combining data from many sources, scientists can put together the phylogeny of an organism; since phylogenetic trees are hypotheses, they will continue to change as new types of life are discovered and new information is learned. Limitations of Phylogenetic Trees It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings or after the evolution of a major new adaptation, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure 20.4 shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs appear more similar than lizards and rabbits. Figure 20.4 This ladder-like phylogenetic tree of vertebrates is rooted by an organism that lacked a vertebral column. At each branch point, organisms with different characters are placed in different groups based on the characteristics they share. This OpenStax book is available for free at http://cnx.org/content/col12078/ |
1.6 Chapter 20 | Phylogenies and the History of Life 811 Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, the length of a branch does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 20.4, the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure 20.4, the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in Figure 20.4, just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative. Head to this website (http://openstaxcollege.org/l/tree_of_life) to see interactive exercises that allow you to explore the evolutionary relationships among species. What is the main function of the ITOL (Interactive Tree of Life) website? a. iTOL is a website that provides the history about the Tree of Life. b. iTOL is a website that provides guidelines for researching data to create a phylogenetic tree. c. iTOL is an online tool that provides the display and manipulation of pre-computed phylogenetic trees, and you can upload and display your own trees and data. d. iTOL is a website that explains the evolutionary relationships among species. Think About It How does a phylogenetic tree relate to the passing of time? What other questions about the evolutionary history of an organism and its relatedness to other organisms can a phylogenetic tree answer? The Levels of Classification Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. Think about how a grocery store is organized. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into |
aisles, then each aisle into categories and brands, and then finally a single product. This organization from larger to smaller, more specific categories is called a hierarchical system. The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species (Figure 20.5). 812 Chapter 20 | Phylogenies and the History of Life Figure 20.5 The taxonomic classification system uses a hierarchical model to organize living organisms into increasingly specific categories. The common dog, Canis lupus familiaris, is a subspecies of Canis lupus, which also includes the wolf and dingo. (credit “dog”: modification of work by Janneke Vreugdenhil) The kingdom Animalia stems from the Eukarya domain. For the common dog, the classification levels would be as shown in Figure 20.5. Therefore, the full name of an organism technically has eight terms. For the dog, it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 813 its two-word scientific name, in what is called binomial nomenclature. Therefore, the scientific name of the dog is Canis lupus. The name at each level is also called a taxon. In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name |
that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are considered separate subspecies due to geographic or behavioral isolation or other factors. Figure 20.6 shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology developed, more precise phylogenies have been determined. 814 Chapter 20 | Phylogenies and the History of Life Figure 20.6 At each sublevel in the taxonomic classification system, organisms become more similar. Dogs and wolves are the same species because they can breed and produce viable offspring, but they are different enough to be classified as different subspecies. (credit “plant”: modification of work by "berduchwal"/Flickr; credit “insect”: modification of work by Jon Sullivan; credit “fish”: modification of work by Christian Mehlführer; credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck; credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog”: modification of work by "digital_image_fan"/Flickr) At what levels are cats and dogs considered to be part of the same group? a. Cats and dogs are only found together in the Domain level. b. Cats and dogs are in the same group beginning at the Domain level and including the sublevels Kingdom, Phylum, Class, and Order. c. Cats and dogs are in the same group beginning at the Family level. d. Cats and dogs are part of the same group beginning with the Order: Carnivora level. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the |
History of Life 815 Visit this website (http://openstaxcollege.org/l/classify_life) to classify three organisms—bear, orchid, and sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the bear or the Launch Interactive button. Using the taxonomic classification system, which Kingdom category best describes a bear? a. Plantae: Multicellular organisms that get their energy through photosynthesis. b. Animalia: Multicellular organismsthat get their energy through ingesting other organisms. c. Fungi: Single-celled and multi-celled organisms that get their energy mainly by absorbing nutrients from their surroundings and not through photosynthesis. Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, changes and updates must be made as new discoveries occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available. In addition, classification historically has focused on grouping organisms mainly by shared characteristics and does not necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example, despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the closest living relative of the whale. 20.2 | Determining Evolutionary Relationships • What is the difference between homologous and analogous traits? How are these traits used when determining evolutionary relatedness? • What is cladistics? How does a cladogram differ from a phylogenetic tree? • What is parsimony? Connection for AP® Courses To build phylogenetic trees, scientists must collect accurate information that allows them to make evolutionary connections among organisms. Using morphological and molecular data, scientists identify both homologous and analogous characteristics and genes. (In a prior chapter we explored the differences between homologous and analogous traits and how they relate to convergent and divergent evolution.) Similarities among organisms stem either from shared ancestral history (homologies) or from separate evolutionary paths (analogies). Cladograms are constructed by using shared derived traits to distinguish different groups of species from one another. For example, lizards, rabbits and humans all descended from a common ancestor that had an amniotic egg; thus, lizards, rabbits, and humans all belong to the same clade. Vertebrata is a larger clade that also includes fish, lamprey, and lancelets. The closer two species |
or groups are located to each on a phylogenetic tree or cladogram, they more recently they shared a common ancestor. With the influx of new information, scientists can revise phylogenetic trees; for example, computer programs, such as one called BLAST, which helps determine relatedness using DNA sequencing. Typically, a phylogenetic tree is constructed with the simplest explanation of evolutionary history (maximum parsimony) and the fewest number of evolutionary steps. Understanding phylogeny extends far beyond understanding the evolutionary history of species on Earth. For botanists, phylogeny acts as a guide to discovering new plants that can be used to make food, medicine, and clothing. For doctors, phylogenies provide information about the origin of diseases and how to treat them, for example, HIV/AIDS. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent 816 Chapter 20 | Phylogenies and the History of Life foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.A Change in the genetic makeup of a population over time is evolution. Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics. Science Practice Learning Objective 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.9 The student is able to evaluate evidence provided by data from many scientific disciplines that support biological evolution. Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics. Science Practice Learning Objective 5.2 The student can refine observations and measurements based on data analysis. 1.10 The student is able to refine evidence based on data from many scientific disciplines that support biological evolution. Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics. Science Practice Learning Objective 4.2 The student can design a plan for collecting data to answer a particular scientific question. 1.11 The student is able to design a plan to answer scientific questions regarding how organisms have changed over time using information from morphology, biochemistry, and geology. Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from |
many disciplines, including mathematics. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 1.12 The student is able to connect scientific evidence from many scientific disciplines to support the modern concept of evolution. Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics. Science Practice Science Practice Learning Objective 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 2.1 The student can justify the selection of a mathematical routine to solve problems. 1.13 The student is able to construct and/or justify mathematical models, diagrams or simulations that represent processes of biological evolution. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 817 Science Practice Learning Objective 3.1 The student can pose scientific questions. 1.14 The student is able to pose scientific questions that correctly identify essential properties of shared, core life processes that provide insight into the history of life on Earth. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 6.1 The student can justify claims with evidence. 1.16 The student is able to justify the scientific claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Essential Knowledge 1.B.2 Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Science |
Practice Learning Objective 3.1 The student can pose scientific questions. 1.17 The student is able to pose scientific questions about a group of organisms whose relatedness is described by a phylogenetic tree or cladogram. Essential Knowledge 1.B.2 Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Science Practice Learning Objective 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.18 The student is able to evaluate evidence provided by a data set in conjunction with a phylogenetic tree or simple cladogram to determine evolutionary history and speciation. Essential Knowledge 1.B.2 Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. Science Practice Science Practice Learning Objective 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 2.1 The student can justify the selection of a mathematical routine to solve problems. 1.19 The student is able to create a phylogenetic tree or simple cladogram that correctly represents evolutionary history and speciation from a provided data set. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.15][APLO 1.16][APLO 1.18][APLO 1.17][APLO 1.19][APLO 1.26] Two Options for Similarities In general, organisms that share similar physical features and genomes tend to be more closely related than those that do not. Such features that overlap both morphologically (in form) and genetically are referred to as homologous structures; they stem from developmental similarities that are based on evolution. For example, the bones in the wings of bats and birds have homologous structures (Figure 20.7). 818 Chapter 20 | Phylogenies and the History of Life Figure 20.7 Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary past. (credit a: modification of work by Steve Hillebrand, USFWS; credit b: modification of work by U.S. DOI BLM) Notice it is not simply a single bone, but rather a grouping of several bones arranged in a similar way. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a |
car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, it would be reasonable to conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms. Misleading Appearances Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly related, but appear very much alike. This usually happens because both organisms were in common adaptations that evolved within similar environmental conditions. When similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is called an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. These are called analogous structures (Figure 20.8). Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin; analogous organs have a similar function. For example, the bones in the front flipper of a whale are homologous to the bones in the human arm. These structures are not analogous. The wings of a butterfly and the wings of a bird are analogous but not homologous. Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 819 Figure 20.8 The (c) wing of a honeybee is similar in shape to a (b) bird wing and (a) bat wing, and it serves the same function. However, the honeybee wing is not composed of bones and has a distinctly different structure and embryonic origin. These wing types (insect versus bat and bird) illustrate an analogy—similar structures that do not share an evolutionary history. (credit a: modification of work by Steve Hillebrand, USFWS; credit b: modification of work by U.S. DOI BLM; credit c: modification of work by Jon Sullivan) This website (http://openstaxcollege.org/l/relationships |
) has several examples to show how appearances can be misleading in understanding the phylogenetic relationships of organisms. James Lake of the UCLA/NASA Astrobiology Institute presented new evidence regarding the evolution of eukaryotic cells. He hypothesized that eukaryotes developed from an endosymbiotic gene fusion between the two other domains of life. What kind of genetic evidence would best support this hypothesis? a. Their mitochondrial DNA resembles that of other eukaryotes. b. The chloroplasts of eukaryotes contain a double cell layer. c. All eukaryotic genes are identical to either Archaea or Bacteria. d. Some eukaryotic genes resemble those of Archaea, while some resemble those of Bacteria and some are unlike the genes of either domain. Molecular Comparisons With the advancement of DNA technology, the area of molecular systematics, which describes the use of information on the molecular level including DNA analysis, has blossomed. New computer programs not only confirm many earlier classified organisms, but also uncover previously made errors. As with physical characteristics, even the DNA sequence can be tricky to read in some cases. For some situations, two very closely related organisms can appear unrelated if a mutation 820 Chapter 20 | Phylogenies and the History of Life occurred that caused a shift in the genetic code. An insertion or deletion mutation would move each nucleotide base over one place, causing two similar codes to appear unrelated. Sometimes two segments of DNA code in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not. For both of these situations, computer technologies have been developed to help identify the actual relationships, and, ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 821 Why Does Phylogeny Matter? Evolutionary biologists could list many reasons why understanding phylogeny is important to everyday life in human society. For botanists, phylogeny acts as a guide to discovering new plants that can be used to benefit people. Think of all the ways humans use plants—food, medicine, and clothing are a few examples. If a plant contains a compound that is effective in treating diseases, scientists might want to examine all of the relatives of that plant for other useful drugs. A research team |
in China identified a segment of DNA thought to be common to some medicinal plants in the family Fabaceae (the legume family) and worked to identify which species had this segment (Figure 20.9). After testing plant species in this family, the team found a DNA marker (a known location on a chromosome that enabled them to identify the species) present. Then, using the DNA to uncover phylogenetic relationships, the team could identify whether a newly discovered plant was in this family and assess its potential medicinal properties. Figure 20.9 Dalbergia sissoo (D. sissoo) is in the Fabaceae, or legume family. Scientists found that D. sissoo shares a DNA marker with species within the Fabaceae family that have antifungal properties. Subsequently, D. sissoo was shown to have fungicidal activity, supporting the idea that DNA markers can be used to screen for plants with potential medicinal properties. 822 Chapter 20 | Phylogenies and the History of Life Part b of the figure shows a hypothetical model of the evolution of the cell membrane of gram-negative bacteria, which has a double membrane. If this hypothesis is true, what does it suggest about the evolution of mitochondria and chloroplasts in eukaryotic cells and why? a. Chloroplasts and mitochondria did not come about through endosymbiosis with gram-negative bacteria because these organelles have a single membrane. b. Chloroplasts and mitochondria likely evolved later in eukaryotic cells, as these organelles show no similarities to prokaryotes. c. Chloroplasts and mitochondria came about through endosymbiosis with Archaea and gram positive bacteria because these organelles have prokaryote-like DNA. d. Chloroplasts and mitochondria came about through endosymbiosis with gram-negative bacteria because these organelles have a double membrane. Building Phylogenetic Trees How do scientists construct phylogenetic trees? After the homologous and analogous traits are sorted, scientists often organize the homologous traits using a system called cladistics. This system sorts organisms into clades: groups of organisms that descended from a single ancestor. For example, in Figure 20.10, all of the organisms in the orange region evolved from a single ancestor that had amniotic eggs. Consequently, all of these organisms also have amniotic eggs and make a single clade, also |
called a monophyletic group. Clades must include all of the descendants from a branch point. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 823 Figure 20.10 Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg. Thus, lizards, rabbits, and humans all belong to the clade Amniota. Vertebrata is a larger clade that also includes fish and lamprey. Eukaryotic mitochondria contain their own DNA, known as mitochondrial DNA, or mtDNA. Nuclear and mitochondrial DNA are thought to be of different (separate) evolutionary origin, with the mitochondrial DNA being derived from the circular chromosomes of bacteria that were engulfed by ancient prokaryotic cells. Does the presence of mitochondrial DNA suggest that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today? Why or why not? a. Yes, because it shows the prokaryotes and eukaryotes use similar organelles, namely, mitochondria. b. Yes, because it suggests the eukaryotes possess traits that were likely conserved from prokaryotic ancestors. c. No, because mitochondrial DNA is very different from the DNA within a eukaryote’s nucleus. d. No, because mitochondrial DNA is not used by the eukaryotic cells. Clades can vary in size depending on which branch point is being referenced. The important factor is that all of the organisms in the clade or monophyletic group stem from a single point on the tree. This can be remembered because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship. Figure 20.11 shows various examples of clades. Notice how each clade comes from a single point, whereas the non-clade groups show branches that do not share a single point. 824 Chapter 20 | Phylogenies and the History of Life Figure 20.11 All the organisms within a clade stem from a single point on the tree. A clade may contain multiple groups, as in the case of animals, fungi and plants, or a single group, as in the case of flagellates. Groups that diverge at a different branch point, or that do not include |
all groups in a single branch point, are not considered clades. Glycolysis, or the breakdown of glucose, is a process used by almost all organisms as a way to release energy stored within glucose molecules. This energy can then be stored in cells as ATP, which powers cell processes when needed. What does this show, in terms of the evolutionary history of cells using glycolysis? a. Glycolysis has been conserved despite the independent evolution of the three domains of life. b. Prokaryotes would likely not benefit from the Krebs cycle or the ETC. c. Prokaryotes likely evolved after eukaryotes. d. Glycolysis is the only way in which living things can break down glucose. Shared Characteristics Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats over and over as one goes through the phylogenetic tree of life: 1. A change in the genetic makeup of an organism leads to a new trait which becomes prevalent in the group. 2. Many organisms descend from this point and have this trait. 3. New variations continue to arise: some are adaptive and persist, leading to new traits. 4. With new traits, a new branch point is determined (go back to step 1 and repeat). If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character because all of the organisms in the taxon or clade have that trait. The vertebrate in Figure 20.10 is a shared ancestral character. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in Figure 20.10 have this trait, and to those that do, it is called a shared derived character because this trait derived at some point but does not include all of the ancestors in the tree. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 825 The tricky aspect to shared ancestral and shared derived characters is the fact that these terms are relative. The same trait can be considered one or the other depending on the particular diagram being used. Returning to Figure 20.10, note that the amniotic egg is a shared ancestral character for the Amniota clade, while having hair is a shared |
derived character for some organisms in this group. These terms help scientists distinguish between clades in the building of phylogenetic trees. Choosing the Right Relationships Imagine being the person responsible for organizing all of the items in a department store properly—an overwhelming task. Organizing the evolutionary relationships of all life on Earth proves much more difficult: scientists must span enormous blocks of time and work with information from long-extinct organisms. Trying to decipher the proper connections, especially given the presence of homologies and analogies, makes the task of building an accurate tree of life extraordinarily difficult. Add to that the advancement of DNA technology, which now provides large quantities of genetic sequences to be used and analyzed. Taxonomy is a subjective discipline: many organisms have more than one connection to each other, so each taxonomist will decide the order of connections. To aid in the tremendous task of describing phylogenies accurately, scientists often use a concept called maximum parsimony, which means that events occurred in the simplest, most obvious way. For example, if a group of people entered a forest preserve to go hiking, based on the principle of maximum parsimony, one could predict that most of the people would hike on established trails rather than forge new ones. For scientists deciphering evolutionary pathways, the same idea is used: the pathway of evolution probably includes the fewest major events that coincide with the evidence at hand. Starting with all of the homologous traits in a group of organisms, scientists look for the most obvious and simple order of evolutionary events that led to the occurrence of those traits. Head to this website (http://openstaxcollege.org/l/32species) to learn how maximum parsimony is used to create phylogenetic trees. What do phylogenetic relationships refer to? a. the similarities among organisms b. the differences among organisms c. the evolution of the shape, size and number of body parts d. the relative times in the past that species shared common ancestors These tools and concepts are only a few of the strategies scientists use to tackle the task of revealing the evolutionary history of life on Earth. Recently, newer technologies have uncovered surprising discoveries with unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer to mapping the evolutionary history of all life on Earth. 826 Chapter 20 | Phylogenies and the History of Life Activity Using a data set provided by your teacher or other sources, |
construct a phylogenetic tree or cladogram to reflect the evolutionary history among a group of organisms based on shared characteristics. Then share the phylogenetic tree or cladogram with peers for review and revision. Lab Investigation AP® Biology Investigative Labs: Inquiry-Based Approach, Investigation 3: Comparing DNA Sequences to Understand Evolutionary Relationships with BLAST. Students will learn to use a common tool, BLAST, to compare several genes from different organisms and then use this information to construct a cladogram to determine evolutionary relatedness among species. Then students will use BLAST to track a gene(s) of choice through several species. Bioinformatics has many applications, including understanding genetic disease. Think About It Why must scientists distinguish between homologous and analogous characteristics before building phylogenetic trees? Do more closely related organisms share homologous or analogous traits? Which type of trait is used to support convergent or divergent evolution? 20.3 | Perspectives on the Phylogenetic Tree In this section, you will explore the following questions: • What is horizontal gene transfer and its significance in constructing phylogenetic trees? • How do prokaryotes and eukaryotes transfer genes horizontally? • What are other models of phylogenetic relationships and how do they differ from the original phylogenetic tree concept? Connection for AP® Courses Newer technologies have uncovered surprising discoveries with unexpected relationships among organisms, such as the fact that humans seems to be more closely related to fungi than fungi are to plants. (Think about that the next time you see a mushroom). As the information about DNA sequences grows, scientists will become closer to mapping a more accurate evolutionary history of all life on Earth. What makes phylogeny difficult, especially among prokaryotes, is the transfer of genes horizontally (horizontal gene transfer, or HGT) between unrelated species. Like mutations, HGT introduces genetic variation into the bacterial population. This passing of genes between species adds a layer of complexity to understanding relatedness. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. This OpenStax book is available for free at http |
://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 827 Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 3.1 The student can pose scientific questions. 1.14 The student is able to pose scientific questions that correctly identify essential properties of shared, core life processes that provide insight into the history of life on Earth. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective Big Idea 3 Enduring Understanding 3.C Essential Knowledge Science Practice Learning Objective 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. Living systems store, retrieve, transmit and respond to information essential to life processes. The processing of genetic information is imperfect and is a source of genetic variation. 3.C.2 Biological systems have multiple processes that increase genetic variation. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.27 The student is able to construct an explanation of processes that increase variation within a population. The concepts of phylogenetic modeling are constantly changing. It is one of the most dynamic fields of study in all of biology. Over the last several decades, new research has challenged scientists’ ideas about how organisms are related. New models of these relationships have been proposed for consideration by the scientific community. Many phylogenetic trees have been shown as models of the evolutionary relationship among species. Phylogenetic trees originated with Charles Darwin, who sketched the first phylogenetic tree in 1837 (Figure 20.12a), which served as a pattern for subsequent studies for more than a century. The concept of a phylogenetic tree with a single trunk representing a common ancestor, with the branches representing the divergence of species from this ancestor, fits well with the structure of many common trees, such as the oak (Figure 20.12b). However, evidence from modern |
DNA sequence analysis and newly developed computer algorithms has caused skepticism about the validity of the standard tree model in the scientific community. 828 Chapter 20 | Phylogenies and the History of Life Figure 20.12 The (a) concept of the “tree of life” goes back to an 1837 sketch by Charles Darwin. Like an (b) oak tree, the “tree of life” has a single trunk and many branches. (credit b: modification of work by "Amada44"/Wikimedia Commons) Limitations to the Classic Model Classical thinking about prokaryotic evolution, included in the classic tree model, is that species evolve clonally. That is, they produce offspring themselves with only random mutations causing the descent into the variety of modern-day and extinct species known to science. This view is somewhat complicated in eukaryotes that reproduce sexually, but the laws of Mendelian genetics explain the variation in offspring, again, to be a result of a mutation within the species. The concept of genes being transferred between unrelated species was not considered as a possibility until relatively recently. Horizontal gene transfer (HGT), also known as lateral gene transfer, is the transfer of genes between unrelated species. HGT has been shown to be an ever-present phenomenon, with many evolutionists postulating a major role for this process in evolution, thus complicating the simple tree model. Genes have been shown to be passed between species which are only distantly related using standard phylogeny, thus adding a layer of complexity to the understanding of phylogenetic relationships. The various ways that HGT occurs in prokaryotes is important to understanding phylogenies. Although at present HGT is not viewed as important to eukaryotic evolution, HGT does occur in this domain as well. Finally, as an example of the ultimate gene transfer, theories of genome fusion between symbiotic or endosymbiotic organisms have been proposed to explain an event of great importance—the evolution of the first eukaryotic cell, without which humans could not have come into existence. Horizontal Gene Transfer Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly related species to share genes, influencing their phenotypes. It is thought that HGT is more prevalent in prokaryotes, but that only about 2% of the prokaryotic genome may be |
transferred by this process. Some researchers believe such estimates are premature: the actual importance of HGT to evolutionary processes must be viewed as a work in progress. As the phenomenon is investigated more thoroughly, it may be revealed to be more common. Many scientists believe that HGT and mutation appear to be (especially in prokaryotes) a significant source of genetic variation, which is the raw material for the process of natural selection. These transfers may occur between any two species that share an intimate relationship (Table 20.1). Summary of Mechanisms of Prokaryotic and Eukaryotic HGT Mechanism Mode of Transmission Example Prokaryotes transformation DNA uptake many prokaryotes transduction bacteriophage (virus) bacteria conjugation pilus many prokaryotes Table 20.1 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 829 Summary of Mechanisms of Prokaryotic and Eukaryotic HGT Mechanism Mode of Transmission Example gene transfer agents phage-like particles purple non-sulfur bacteria Eukaryotes from food organisms unknown aphid jumping genes transposons rice and millet plants epiphytes/parasites unknown yew tree fungi from viral infections Table 20.1 HGT in Prokaryotes The mechanism of HGT has been shown to be quite common in the prokaryotic domains of Bacteria and Archaea, significantly changing the way their evolution is viewed. The majority of evolutionary models, such as in the Endosymbiont Theory, propose that eukaryotes descended from multiple prokaryotes, which makes HGT all the more important to understanding the phylogenetic relationships of all extant and extinct species. The fact that genes are transferred among common bacteria is well known to microbiology students. These gene transfers between species are the major mechanism whereby bacteria acquire resistance to antibiotics. Classically, this type of transfer has been thought to occur by three different mechanisms: 1. Transformation: naked DNA is taken up by a bacteria 2. Transduction: genes are transferred using a virus 3. Conjugation: the use a hollow tube called a pilus to transfer genes between organisms More recently, a fourth mechanism of gene transfer between prokaryotes has been discovered. Small, virus-like particles called gene transfer agents (GTAs) transfer random genomic segments from one species of pro |
karyote to another. GTAs have been shown to be responsible for genetic changes, sometimes at a very high frequency compared to other evolutionary processes. The first GTA was characterized in 1974 using purple, non-sulfur bacteria. These GTAs, which are thought to be bacteriophages that lost the ability to reproduce on their own, carry random pieces of DNA from one organism to another. The ability of GTAs to act with high frequency has been demonstrated in controlled studies using marine bacteria. Gene transfer events in marine prokaryotes, either by GTAs or by viruses, have been estimated to be as high as 1013 per year in the Mediterranean Sea alone. GTAs and viruses are thought to be efficient HGT vehicles with a major impact on prokaryotic evolution. As a consequence of this modern DNA analysis, the idea that eukaryotes evolved directly from Archaea has fallen out of favor. While eukaryotes share many features that are absent in bacteria, such as the TATA box (found in the promoter region of many genes), the discovery that some eukaryotic genes were more homologous with bacterial DNA than Archaea DNA made this idea less tenable. Furthermore, the fusion of genomes from Archaea and Bacteria by endosymbiosis has been proposed as the ultimate event in eukaryotic evolution. HGT in Eukaryotes Although it is easy to see how prokaryotes exchange genetic material by HGT, it was initially thought that this process was absent in eukaryotes. After all, prokaryotes are but single cells exposed directly to their environment, whereas the sex cells of multicellular organisms are usually sequestered in protected parts of the body. It follows from this idea that the gene transfers between multicellular eukaryotes should be more difficult. Indeed, it is thought that this process is rarer in eukaryotes and has a much smaller evolutionary impact than in prokaryotes. In spite of this fact, HGT between distantly related organisms has been demonstrated in several eukaryotic species, and it is possible that more examples will be discovered in the future. In plants, gene transfer has been observed in species that cannot cross-pollinate by normal means. Transposons or “jumping genes” have been shown to transfer between rice and millet plant species. Furthermore, fungal species feeding on yew trees, from which the anti-cancer drug paclitaxel is |
derived from the bark, have acquired the ability to make paclitaxel themselves, a clear example of gene transfer. In animals, a particularly interesting example of HGT occurs within the aphid species (Figure 20.13). Aphids are insects that vary in color based on carotenoid content. Carotenoids are pigments made by a variety of plants, fungi, and microbes, and they serve a variety of functions in animals, who obtain these chemicals from their food. Humans require carotenoids 830 Chapter 20 | Phylogenies and the History of Life to synthesize vitamin A, and we obtain them by eating orange fruits and vegetables: carrots, apricots, mangoes, and sweet potatoes. On the other hand, aphids have acquired the ability to make the carotenoids on their own. According to DNA analysis, this ability is due to the transfer of fungal genes into the insect by HGT, presumably as the insect consumed fungi for food. A carotenoid enzyme called a desaturase is responsible for the red coloration seen in certain aphids, and it has been further shown that when this gene is inactivated by mutation, the aphids revert back to their more common green color (Figure 20.13). Figure 20.13 (a) Red aphids get their color from red carotenoid pigment. Genes necessary to make this pigment are present in certain fungi, and scientists speculate that aphids acquired these genes through HGT after consuming fungi for food. If genes for making carotenoids are inactivated by mutation, the aphids revert back to (b) their green color. Red coloration makes the aphids a lot more conspicuous to predators, but evidence suggests that red aphids are more resistant to insecticides than green ones. Thus, red aphids may be more fit to survive in some environments than green ones. (credit a: modification of work by Benny Mazur; credit b: modification of work by Mick Talbot) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 20 | Phylogenies and the History of Life 831 Barbara McClintock (1902–1992) discovered transposons while working on maize genetics. Figure 20.14 What does the Eukaryote-first hypothesis suggest? a. b. c. d. that mitochondria were first established in a prokaryotic host which |
acquired a nucleus to become the first eukaryotic cell that the nucleus evolved in prokaryotes first followed by fusion of the new eukaryote with bacteria that became mitochondria that prokaryotes actually evolved from eukaryotes by losing genes and complexity that eukaryotes developed Golgi before mitochondria Genome Fusion and the Evolution of Eukaryotes Scientists believe the ultimate in HGT occurs through genome fusion between different species of prokaryotes when two symbiotic organisms become endosymbiotic. This occurs when one species is taken inside the cytoplasm of another species, which ultimately results in a genome consisting of genes from both the endosymbiont and the host. This mechanism is an aspect of the Endosymbiont Theory, which is accepted by a majority of biologists as the mechanism whereby eukaryotic cells obtained their mitochondria and chloroplasts. However, the role of endosymbiosis in the development of the nucleus is more controversial. Nuclear and mitochondrial DNA are thought to be of different (separate) evolutionary origin, with the mitochondrial DNA being derived from the circular genomes of bacteria that were engulfed by ancient prokaryotic cells. Mitochondrial DNA can be regarded as the smallest chromosome. Interestingly enough, mitochondrial DNA is inherited only from the mother. The mitochondrial DNA degrades in sperm when the sperm degrades in the fertilized egg or in other instances when the mitochondria located in the flagellum of the sperm fails to enter the egg. Within the past decade, the process of genome fusion by endosymbiosis has been proposed by James Lake of the UCLA/ NASA Astrobiology Institute to be responsible for the evolution of the first eukaryotic cells (Figure 20.15a). Using DNA analysis and a new mathematical algorithm called conditioned reconstruction (CR), his laboratory proposed that eukaryotic cells developed from an endosymbiotic gene fusion between two species, one an Archaea and the other a Bacteria. As mentioned, some eukaryotic genes resemble those of Archaea, whereas others resemble those from Bacteria. An endosymbiotic fusion event, such as Lake has proposed, would clearly explain this observation. On the other hand, this work is new and the CR algorithm is relatively unsubstantiated, which causes many scientists to resist this hypothesis. More recent work by Lake (Figure 20.15b) proposes that gram-negative bacteria, which are unique within their domain 832 |
Chapter 20 | Phylogenies and the History of Life in that they contain two lipid bilayer membranes, indeed resulted from an endosymbiotic fusion of archaeal and bacterial species. The double membrane would be a direct result of the endosymbiosis, with the endosymbiont picking up the second membrane from the host as it was internalized. This mechanism has also been used to explain the double membranes found in mitochondria and chloroplasts. Lake’s work is not without skepticism, and the ideas are still debated within the biological science community. In addition to Lake’s hypothesis, there are several other competing theories as to the origin of eukaryotes. How did the eukaryotic nucleus evolve? One theory is that the prokaryotic cells produced an additional membrane that surrounded the bacterial chromosome. Some bacteria have the DNA enclosed by two membranes; however, there is no evidence of a nucleolus or nuclear pores. Other proteobacteria also have membrane-bound chromosomes. If the eukaryotic nucleus evolved this way, we would expect one of the two types of prokaryotes to be more closely related to eukaryotes. Figure 20.15 The theory that mitochondria and chloroplasts are endosymbiotic in origin is now widely accepted. More controversial is the proposal that (a) the eukaryotic nucleus resulted from the fusion of archaeal and bacterial genomes, and that (b) Gram-negative bacteria, which have two membranes, resulted from the fusion of Archaea and Grampositive bacteria, each of which has a single membrane. The nucleus-first hypothesis proposes that the nucleus evolved in prokaryotes first (Figure 20.16a), followed by a later fusion of the new eukaryote with bacteria that became mitochondria. The mitochondria-first hypothesis proposes that mitochondria were first established in a prokaryotic host (Figure 20.16b), which subsequently acquired a nucleus, by fusion or other mechanisms, to become the first eukaryotic cell. Most interestingly, the eukaryote-first hypothesis proposes that prokaryotes actually evolved from eukaryotes by losing genes and complexity (Figure 20.16c). All of these hypotheses are testable. Only time and more experimentation will determine which hypothesis is best supported by data. This OpenStax book is available for free at http://cnx.org/content/col12078/1 |
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