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convergent evolution : process by which groups of organisms independently evolve to similar forms | https://openstax.org/books/biology/pages/18-key-terms |
dispersal : allopatric speciation that occurs when a few members of a species move to a new geographical area | https://openstax.org/books/biology/pages/18-key-terms |
divergent evolution : process by which groups of organisms evolve in diverse directions from a common point | https://openstax.org/books/biology/pages/18-key-terms |
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 | https://openstax.org/books/biology/pages/18-key-terms |
gradual speciation model : model that shows how species diverge gradually over time in small steps | https://openstax.org/books/biology/pages/18-key-terms |
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 | https://openstax.org/books/biology/pages/18-key-terms |
homologous structures : parallel structures in diverse organisms that have a common ancestor | https://openstax.org/books/biology/pages/18-key-terms |
hybrid : offspring of two closely related individuals, not of the same species | https://openstax.org/books/biology/pages/18-key-terms |
hybrid zone : area where two closely related species continue to interact and reproduce, forming hybrids | https://openstax.org/books/biology/pages/18-key-terms |
natural selection : reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change | https://openstax.org/books/biology/pages/18-key-terms |
postzygotic barrier : reproductive isolation mechanism that occurs after zygote formation | https://openstax.org/books/biology/pages/18-key-terms |
prezygotic barrier : reproductive isolation mechanism that occurs before zygote formation | https://openstax.org/books/biology/pages/18-key-terms |
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 | https://openstax.org/books/biology/pages/18-key-terms |
reinforcement : continued speciation divergence between two related species due to low fitness of hybrids between them | https://openstax.org/books/biology/pages/18-key-terms |
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 | https://openstax.org/books/biology/pages/18-key-terms |
speciation : formation of a new species | https://openstax.org/books/biology/pages/18-key-terms |
species : group of populations that interbreed and produce fertile offspring | https://openstax.org/books/biology/pages/18-key-terms |
sympatric speciation : speciation that occurs in the same geographic space | https://openstax.org/books/biology/pages/18-key-terms |
temporal isolation : differences in breeding schedules that can act as a form of prezygotic barrier leading to reproductive isolation | https://openstax.org/books/biology/pages/18-key-terms |
variation : genetic differences among individuals in a population | https://openstax.org/books/biology/pages/18-key-terms |
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 | https://openstax.org/books/biology/pages/18-key-terms |
vicariance : allopatric speciation that occurs when something in the environment separates organisms of the same species into separate groups | https://openstax.org/books/biology/pages/18-key-terms |
The modern synthesis of evolutionary theory grew out of the cohesion of Darwinâs, Wallaceâs, and Mendelâs 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 populationsâ 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. | https://openstax.org/books/biology/pages/19-chapter-summary |
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 individualâs 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 individualâs 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. | https://openstax.org/books/biology/pages/19-chapter-summary |
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 populationâs variation. Directional selection works to shift a populationâs 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. | https://openstax.org/books/biology/pages/19-chapter-summary |
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. | https://openstax.org/books/biology/pages/19-chapter-summary |
adaptive evolution : increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection | https://openstax.org/books/biology/pages/19-key-terms |
allele frequency : (also, gene frequency) rate at which a specific allele appears within a population | https://openstax.org/books/biology/pages/19-key-terms |
assortative mating : when individuals tend to mate with those who are phenotypically similar to themselves | https://openstax.org/books/biology/pages/19-key-terms |
bottleneck effect : magnification of genetic drift as a result of natural events or catastrophes | https://openstax.org/books/biology/pages/19-key-terms |
cline : gradual geographic variation across an ecological gradient | https://openstax.org/books/biology/pages/19-key-terms |
directional selection : selection that favors phenotypes at one end of the spectrum of existing variation | https://openstax.org/books/biology/pages/19-key-terms |
diversifying selection : selection that favors two or more distinct phenotypes | https://openstax.org/books/biology/pages/19-key-terms |
evolutionary fitness : (also, Darwinian fitness) individualâs ability to survive and reproduce | https://openstax.org/books/biology/pages/19-key-terms |
founder effect : event that initiates an allele frequency change in part of the population, which is not typical of the original population | https://openstax.org/books/biology/pages/19-key-terms |
frequency-dependent selection : selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) | https://openstax.org/books/biology/pages/19-key-terms |
gene flow : flow of alleles in and out of a population due to the migration of individuals or gametes | https://openstax.org/books/biology/pages/19-key-terms |
gene pool : all of the alleles carried by all of the individuals in the population | https://openstax.org/books/biology/pages/19-key-terms |
genetic drift : effect of chance on a populationâs gene pool | https://openstax.org/books/biology/pages/19-key-terms |
genetic structure : distribution of the different possible genotypes in a population | https://openstax.org/books/biology/pages/19-key-terms |
genetic variance : diversity of alleles and genotypes in a population | https://openstax.org/books/biology/pages/19-key-terms |
geographical variation : differences in the phenotypic variation between populations that are separated geographically | https://openstax.org/books/biology/pages/19-key-terms |
good genes hypothesis : theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease | https://openstax.org/books/biology/pages/19-key-terms |
handicap principle : theory of sexual selection that argues only the fittest individuals can afford costly traits | https://openstax.org/books/biology/pages/19-key-terms |
heritability : fraction of population variation that can be attributed to its genetic variance | https://openstax.org/books/biology/pages/19-key-terms |
honest signal : trait that gives a truthful impression of an individualâs fitness | https://openstax.org/books/biology/pages/19-key-terms |
inbreeding : mating of closely related individuals | https://openstax.org/books/biology/pages/19-key-terms |
inbreeding depression : increase in abnormalities and disease in inbreeding populations | https://openstax.org/books/biology/pages/19-key-terms |
macroevolution : broader scale evolutionary changes seen over paleontological time | https://openstax.org/books/biology/pages/19-key-terms |
microevolution : changes in a populationâs genetic structure | https://openstax.org/books/biology/pages/19-key-terms |
modern synthesis : overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today | https://openstax.org/books/biology/pages/19-key-terms |
nonrandom mating : changes in a populationâs gene pool due to mate choice or other forces that cause individuals to mate with certain phenotypes more than others | https://openstax.org/books/biology/pages/19-key-terms |
population genetics : study of how selective forces change the allele frequencies in a population over time | https://openstax.org/books/biology/pages/19-key-terms |
population variation : distribution of phenotypes in a population | https://openstax.org/books/biology/pages/19-key-terms |
relative fitness : individualâs ability to survive and reproduce relative to the rest of the population | https://openstax.org/books/biology/pages/19-key-terms |
selective pressure : environmental factor that causes one phenotype to be better than another | https://openstax.org/books/biology/pages/19-key-terms |
sexual dimorphism : phenotypic difference between the males and females of a population | https://openstax.org/books/biology/pages/19-key-terms |
stabilizing selection : selection that favors average phenotypes | https://openstax.org/books/biology/pages/19-key-terms |
Scientists continually gain new information that helps understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships. | https://openstax.org/books/biology/pages/20-chapter-summary |
To build phylogenetic trees, scientists must collect accurate information that allows them to make evolutionary connections between organisms. Using morphologic and molecular data, scientists work to identify homologous characteristics and genes. Similarities between organisms can stem either from shared evolutionary history (homologies) or from separate evolutionary paths (analogies). Newer technologies can be used to help distinguish homologies from analogies. After homologous information is identified, scientists use cladistics to organize these events as a means to determine an evolutionary timeline. Scientists apply the concept of maximum parsimony, which states that the order of events probably occurred in the most obvious and simple way with the least amount of steps. For evolutionary events, this would be the path with the least number of major divergences that correlate with the evidence. | https://openstax.org/books/biology/pages/20-chapter-summary |
The phylogenetic tree, first used by Darwin, is the classic âtree of lifeâ model describing phylogenetic relationships among species, and the most common model used today. New ideas about HGT and genome fusion have caused some to suggest revising the model to resemble webs or rings. | https://openstax.org/books/biology/pages/20-chapter-summary |
analogy : (also, homoplasy) characteristic that is similar between organisms by convergent evolution, not due to the same evolutionary path | https://openstax.org/books/biology/pages/20-key-terms |
basal taxon : branch on a phylogenetic tree that has not diverged significantly from the root ancestor | https://openstax.org/books/biology/pages/20-key-terms |
binomial nomenclature : system of two-part scientific names for an organism, which includes genus and species names | https://openstax.org/books/biology/pages/20-key-terms |
branch point : node on a phylogenetic tree where a single lineage splits into distinct new ones | https://openstax.org/books/biology/pages/20-key-terms |
cladistics : system used to organize homologous traits to describe phylogenies | https://openstax.org/books/biology/pages/20-key-terms |
class : division of phylum in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
eukaryote-first hypothesis : proposal that prokaryotes evolved from eukaryotes | https://openstax.org/books/biology/pages/20-key-terms |
family : division of order in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
gene transfer agent (GTA) : bacteriophage-like particle that transfers random genomic segments from one species of prokaryote to another | https://openstax.org/books/biology/pages/20-key-terms |
genome fusion : fusion of two prokaryotic genomes, presumably by endosymbiosis | https://openstax.org/books/biology/pages/20-key-terms |
genus : division of family in the taxonomic classification system; the first part of the binomial scientific name | https://openstax.org/books/biology/pages/20-key-terms |
horizontal gene transfer (HGT) : (also, lateral gene transfer) transfer of genes between unrelated species | https://openstax.org/books/biology/pages/20-key-terms |
kingdom : division of domain in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
maximum parsimony : applying the simplest, most obvious way with the least number of steps | https://openstax.org/books/biology/pages/20-key-terms |
mitochondria-first hypothesis : proposal that prokaryotes acquired a mitochondrion first, followed by nuclear development | https://openstax.org/books/biology/pages/20-key-terms |
molecular systematics : technique using molecular evidence to identify phylogenetic relationships | https://openstax.org/books/biology/pages/20-key-terms |
monophyletic group : (also, clade) organisms that share a single ancestor | https://openstax.org/books/biology/pages/20-key-terms |
nucleus-first hypothesis : proposal that prokaryotes acquired a nucleus first, and then the mitochondrion | https://openstax.org/books/biology/pages/20-key-terms |
order : division of class in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
phylogenetic tree : diagram used to reflect the evolutionary relationships among organisms or groups of organisms | https://openstax.org/books/biology/pages/20-key-terms |
phylogeny : evolutionary history and relationship of an organism or group of organisms | https://openstax.org/books/biology/pages/20-key-terms |
phylum : (plural: phyla) division of kingdom in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
polytomy : branch on a phylogenetic tree with more than two groups or taxa | https://openstax.org/books/biology/pages/20-key-terms |
ring of life : phylogenetic model where all three domains of life evolved from a pool of primitive prokaryotes | https://openstax.org/books/biology/pages/20-key-terms |
rooted : single ancestral lineage on a phylogenetic tree to which all organisms represented in the diagram relate | https://openstax.org/books/biology/pages/20-key-terms |
shared ancestral character : describes a characteristic on a phylogenetic tree that is shared by all organisms on the tree | https://openstax.org/books/biology/pages/20-key-terms |
shared derived character : describes a characteristic on a phylogenetic tree that is shared only by a certain clade of organisms | https://openstax.org/books/biology/pages/20-key-terms |
sister taxa : two lineages that diverged from the same branch point | https://openstax.org/books/biology/pages/20-key-terms |
systematics : field of organizing and classifying organisms based on evolutionary relationships | https://openstax.org/books/biology/pages/20-key-terms |
taxon : (plural: taxa) single level in the taxonomic classification system | https://openstax.org/books/biology/pages/20-key-terms |
taxonomy : science of classifying organisms | https://openstax.org/books/biology/pages/20-key-terms |
web of life : phylogenetic model that attempts to incorporate the effects of horizontal gene transfer on evolution | https://openstax.org/books/biology/pages/20-key-terms |
Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNAânever bothâand they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA. | https://openstax.org/books/biology/pages/21-chapter-summary |
Viral replication within a living cell always produces changes in the cell, sometimes resulting in cell death and sometimes slowly killing the infected cells. There are six basic stages in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release. A viral infection may be productive, resulting in new virions, or nonproductive, which means that the virus remains inside the cell without producing new virions. Bacteriophages are viruses that infect bacteria. They have two different modes of replication: the lytic cycle, where the virus replicates and bursts out of the bacteria, and the lysogenic cycle, which involves the incorporation of the viral genome into the bacterial host genome. Animal viruses cause a variety of infections, with some causing chronic symptoms (hepatitis C), some intermittent symptoms (latent viruses such a herpes simplex virus 1), and others that cause very few symptoms, if any (human herpesviruses 6 and 7). Oncogenic viruses in animals have the ability to cause cancer by interfering with the regulation of the host cell cycle. Viruses of plants are responsible for significant economic damage in both agriculture and plants used for ornamentation. | https://openstax.org/books/biology/pages/21-chapter-summary |
Viruses cause a variety of diseases in humans. Many of these diseases can be prevented by the use of viral vaccines, which stimulate protective immunity against the virus without causing major disease. Viral vaccines may also be used in active viral infections, boosting the ability of the immune system to control or destroy the virus. A series of antiviral drugs that target enzymes and other protein products of viral genes have been developed and used with mixed success. Combinations of anti-HIV drugs have been used to effectively control the virus, extending the lifespans of infected individuals. Viruses have many uses in medicines, such as in the treatment of genetic disorders, cancer, and bacterial infections. | https://openstax.org/books/biology/pages/21-chapter-summary |
Prions are infectious agents that consist of protein, but no DNA or RNA, and seem to produce their deadly effects by duplicating their shapes and accumulating in tissues. They are thought to contribute to several progressive brain disorders, including mad cow disease and Creutzfeldt-Jakob disease. Viroids are single-stranded RNA pathogens that infect plants. Their presence can have a severe impact on the agriculture industry. | https://openstax.org/books/biology/pages/21-chapter-summary |
acellular : lacking cells | https://openstax.org/books/biology/pages/21-key-terms |
acute disease : disease where the symptoms rise and fall within a short period of time | https://openstax.org/books/biology/pages/21-key-terms |
asymptomatic disease : disease where there are no symptoms and the individual is unaware of being infected unless lab tests are performed | https://openstax.org/books/biology/pages/21-key-terms |
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