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Hooray! Where would all of our 5-10 year olds be if it weren't for the Triassic-Jurassic extinction? Statistical analysis suggests some complications in figuring out what the story is here, but a drop in speciation rather than an increase in extinction might have caused this drop in biodiversity, too. The fifth and pos... | Biodiversity and extinction, then and now.mp3 |
The fifth and possibly most famous of all the Big Five extinctions is the Cretaceous-Paleogene event 65 to 66 million years ago. And we know the story. A kablooey, a bolide impact, with dinosaurs dying out in the aftermath, along with lots of other groups. About 75% of all known species went extinct, but this was follo... | Biodiversity and extinction, then and now.mp3 |
About 75% of all known species went extinct, but this was followed by rapid recovery, giving way to what many scientists refer to as the modern biota, including the rise of mammals. And I think we can learn something from that, too. Life is amazingly tenacious. For all the delicacy of an individual or a species, life r... | Biodiversity and extinction, then and now.mp3 |
For all the delicacy of an individual or a species, life recovered in impressive ways at least five different times. Which leads to a fascinating question. What happens after a mass extinction? What are the contingencies, as evolutionists call them, that happen when ecosystems are suddenly opened up by the removal of m... | Biodiversity and extinction, then and now.mp3 |
What are the contingencies, as evolutionists call them, that happen when ecosystems are suddenly opened up by the removal of many species? Who comes next? Who can take advantage of the changes in the openings? And how fast can all that happen? Those are difficult but very interesting and very relevant scientific questi... | Biodiversity and extinction, then and now.mp3 |
And how fast can all that happen? Those are difficult but very interesting and very relevant scientific questions. So there are lots of people working on them because, as I like to say, the past is where you came from. Without knowing where you came from, sometimes you can't know where you're going. And lots of the ext... | Biodiversity and extinction, then and now.mp3 |
Without knowing where you came from, sometimes you can't know where you're going. And lots of the extinction causes I just described should sound a bit familiar, because they bring us back full circle to the beginning of this video in thinking about a sixth mass extinction during the Anthropocene. For the first time in... | Biodiversity and extinction, then and now.mp3 |
And that's the speed and the precision with which this is actually happening. As we talked about in that video, it feels pretty complex. You have this topoisomerase that's unwinding things, the helicase is unzipping it. Then you have the polymerase that can only go from the five prime to three prime direction. It needs... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
Then you have the polymerase that can only go from the five prime to three prime direction. It needs a little primer to get started, but then it starts adding the nucleotides on the lagging strand. You have to have the RNA primer, but then it's going from, once again, from five prime to three prime. So you have these O... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
So you have these Okazaki fragments. And all of this craziness that's happening, and remember, these things don't have brains. These aren't computers. They don't know exactly where to go. It's all because of the chemistry. They're all bumping into each other and reacting in just the right way to make this incredible th... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
They don't know exactly where to go. It's all because of the chemistry. They're all bumping into each other and reacting in just the right way to make this incredible thing happen. Now what I'm about to tell you is really going to boggle your mind because this is happening incredibly fast. DNA polymerase has been clock... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
Now what I'm about to tell you is really going to boggle your mind because this is happening incredibly fast. DNA polymerase has been clocked, at least in E. coli, has a clock at approaching 1,000 base pairs per second. I think the number that I saw was 700-something base pairs per second. So polymerase, let me write t... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
So polymerase, let me write this down. This is worth writing down because it's mind-boggling. It gives you a sense of just how amazing the machinery in your cells are. So it's been as high as, and it can change. It can speed up and slow down, and that's actually been observed. But polymerase, as fast as, as fast as 700... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
So it's been as high as, and it can change. It can speed up and slow down, and that's actually been observed. But polymerase, as fast as, as fast as 700-plus base pairs per second. Base pairs per second. So on this diagram, hey man, it's just zipping. It's just zipping along, at least from our perceptual frame of refer... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
Base pairs per second. So on this diagram, hey man, it's just zipping. It's just zipping along, at least from our perceptual frame of reference. A second seems like a very short amount of time to us, but on a molecular scale, these things are just bouncing around and just getting this stuff done. Now the second thing t... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
A second seems like a very short amount of time to us, but on a molecular scale, these things are just bouncing around and just getting this stuff done. Now the second thing that you might be wondering, okay, this is happening fast, but surely it has a lot of errors. Well, the first thing you might say, well, if it had... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
And so lucky for us that this is actually a fairly precise process. Even in the first pass of the polymerase, you have one mistake, you have one mistake, let me write this down because it's amazing, one mistake for every approximately 10 to the seventh. So this is 10 million nucleotides. And that might seem pretty accu... | Speed and precision of DNA replication Biomolecules MCAT Khan Academy.mp3 |
Why did they go extinct? When did they go extinct? And we know that dinosaurs went extinct about 65 million years ago, right at the end of the Cretaceous. We know there was a big asteroid that hit the planet at that time. We know there was massive volcanic eruptions going on at that time. But it's remained a bit of a m... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
We know there was a big asteroid that hit the planet at that time. We know there was massive volcanic eruptions going on at that time. But it's remained a bit of a mystery. Did one or both of those things cause dinosaurs to go extinct? Or were there other factors involved? We have a paper coming out in Nature Communica... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
Did one or both of those things cause dinosaurs to go extinct? Or were there other factors involved? We have a paper coming out in Nature Communications on the dinosaur extinction and how dinosaurs were changing during those 10 to 12 million years right before their extinction. And this is a collaboration between mysel... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
And this is a collaboration between myself and Mark Norell, my advisor here at the American Museum, and our colleagues Richard Butler and Albert Prieto Marquez in Munich. So what we've done with this project is we've looked specifically at dinosaur anatomy. There has been a lot of previous work on the dinosaur extincti... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
But what most people have done before is they've looked at species counts. They've looked at dinosaur diversity in terms of how many species of dinosaurs there were and how that changed over time. What we do in this paper is that we take a completely different approach. So we're not as interested in the number of speci... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
So we're not as interested in the number of species as we are in the number of kinds. What we've done is we've tried to tease that signal out to look at the physical difference among different dinosaur species, how that represents itself as one comes up to the terminal Cretaceous event. Our results show really that din... | Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction.mp3 |
We have thought about the number of electrons and protons and neutrons and the electron configuration of atoms. But atoms don't just operate in isolation. If that were the case, the whole universe, including us, would just be a bunch of atoms drifting around. What begins to be interesting is how the atoms actually inte... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
What begins to be interesting is how the atoms actually interact with each other. And one of the most interesting forms of interaction is when they stick to each other in some way, shape, or form. And this sticking together of atoms is what we are going to study in this video. Another way to talk about it is how do ato... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
Another way to talk about it is how do atoms bond? Now, as we will see, there are several types of bonds, and it's really a spectrum. But let's just start with what I would consider one of the more extreme type of bonds. And to understand it, let's get a periodic table of elements out right over here. So let's say that... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
And to understand it, let's get a periodic table of elements out right over here. So let's say that we are dealing with a group one element, let's say sodium right over here. What's interesting about group one elements is that they have one valence electron. If we want to visualize the valence electrons for, say, sodiu... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
If we want to visualize the valence electrons for, say, sodium, we could do it with what's known as a Lewis dot structure or a Lewis electron dot structure, sometimes just called a dot structure for short. But because a neutral sodium has one valence electron, we would just draw that one valence electron like that. Now... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
Chlorine is a halogen. Halogens have seven valence electrons. So chlorine's valence electrons would look like this. It has one, two, three, four, five, six, seven valence electrons. And so you can imagine chlorine would love to get another electron in order to complete its outer shell. And we've also studied in other v... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
It has one, two, three, four, five, six, seven valence electrons. And so you can imagine chlorine would love to get another electron in order to complete its outer shell. And we've also studied in other videos these atoms, these elements at the top right of the periodic table, which are not the noble gases, but especia... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
They like to pull electrons, hog electrons. And so what do you think is going to happen when you put these characters together? This guy wants to lose the electrons and chlorine wants to gain an electron. Well, maybe the chlorine will take an electron from the sodium. Now, in a real chemical reaction, you would have tr... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
Well, maybe the chlorine will take an electron from the sodium. Now, in a real chemical reaction, you would have trillions of these and they're bouncing around and different things are happening. But just for simplicity, let's just imagine that these are the only two. And let's imagine that this chlorine is able to nab... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
And let's imagine that this chlorine is able to nab an electron from this sodium. So what is going to happen? Well, this sodium is then going to become positively charged, because it's going to lose an electron. And then the chlorine, the chlorine is now going to gain an electron. So it's going to become a chloride ani... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
And then the chlorine, the chlorine is now going to gain an electron. So it's going to become a chloride anion. Anion is a negative ion. It's a sodium cation, a positive ion. Ion means it's charged. And now it's a chloride anion. So it has the valence electrons that it had before. | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
It's a sodium cation, a positive ion. Ion means it's charged. And now it's a chloride anion. So it has the valence electrons that it had before. And then you could imagine that it gains one from the sodium. And now it has a negative charge. Now, what do we know about positively charged ions and negatively charged ions? | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
So it has the valence electrons that it had before. And then you could imagine that it gains one from the sodium. And now it has a negative charge. Now, what do we know about positively charged ions and negatively charged ions? Well, opposites attract, Coulomb forces. So these two characters are going to be attracted t... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
Now, what do we know about positively charged ions and negatively charged ions? Well, opposites attract, Coulomb forces. So these two characters are going to be attracted to each other. Or another way to think of it, they're gonna stick together. Or another way you could think about it, they are going to be bonded. And... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
Or another way to think of it, they're gonna stick together. Or another way you could think about it, they are going to be bonded. And they will form a compound of sodium chloride. And notice, the whole compound here is neutral. It has a plus one charge for the sodium, a negative one charge for the chloride. But taken ... | Ionic bonds Molecular and ionic compound structure and properties AP Chemistry Khan Academy.mp3 |
We are now going to talk about what is perhaps the most important macromolecule in life, and that is known as nucleic acid. Now first of all, where does that name come from? Well, scientists first observed this in the nucleus of cells, and so that's where you get the nucleic part, and it has some acidic properties, and... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
And perhaps the most famous of the nucleic acids is deoxyribonucleic acid, or DNA for short, and we'll go into some depth in this as we go through our journey in biology, but you might already know that this is the molecule that stores our hereditary information. This DNA, to a large degree, makes you you, and it's kno... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
Just to get a sense of it, you can see right over here the double helix of DNA, where you have one side of your helix right over there, and then another one right over here, and then you kind of have these rungs of this twisted ladder. A DNA molecule, let's say in the human genome, a chromosome, for example, is primari... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
Nucleotides. And we see some examples of nucleotides right over here. This is deoxyadenosine monophosphate, which would be a nucleotide found in DNA. You can see the various parts of it. You have a phosphate group right over here. You have a five-carbon sugar, which in this case is ribose, and then you have what is kno... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
You can see the various parts of it. You have a phosphate group right over here. You have a five-carbon sugar, which in this case is ribose, and then you have what is known as a nitrogenous base, and why is it called nitrogenous? Well, those blue circles represent nitrogen, and we've seen this before. The grays are car... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
Well, those blue circles represent nitrogen, and we've seen this before. The grays are carbons, and the reds are oxygens, and the whites are hydrogens, and so this part of the molecule has some basic characteristics, while this phosphate group at the end, this has some acidic characteristics, and what happens is is the... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
And the way that DNA stores information is every one of these nitrogenous bases right over here, this is adenine, it has a complementary nitrogenous base on the other to complete that rung of the ladder. So adenine matches with thymine in DNA, and we'll see in future videos in RNA, it's a nitrogenous base known as urac... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
We'll go into some depth in this in future videos when we talk about DNA and how information is stored in it, but for the sake of this video, just appreciate that the monomer for a nucleic acid like DNA is a nucleotide, so monomer. And to be very clear, this would not be the only monomer. The analogous nucleotide in RN... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
You can see the difference between the two, that we have an oxygen right over here and we don't have an oxygen right over here. That's why this is called deoxy, and that's why it's deoxyribonucleic acid. You're missing one of those oxygens on your five-carbon sugar but adenine, as I mentioned, is not the only nitrogeno... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
You could have a nucleotide where the nitrogenous base is thymine, and so once again, this looks very similar, but notice what is going on over here. You could have a nucleotide that looks like this. Once again, you have your five-carbon sugar here, you have your phosphate group, but the nitrogenous base here keeps on ... | Introduction to nucleic acids and nucleotides High school biology Khan Academy (2).mp3 |
We've done many videos on Khan Academy on evolution and natural selection, explaining them, but I thought I would do a video going a little bit more in depth in evidence for evolution and natural selection. And I'm starting with this quote, "'Nothing in biology makes sense "'except in light of evolution.'" This is by T... | Evidence for evolution Biology Khan Academy (2).mp3 |
And what he's saying is absolutely true, and this is why it's so important to appreciate the evidence for evolution and natural selection, and to understand them, because before the theory of evolution, biology was just about observation and classification without having a cohesive narrative for how all of this came ab... | Evidence for evolution Biology Khan Academy (2).mp3 |
We have our tools of microbiology and genetics, which gives us even stronger evidence. So a lot of times people say, oh, it's theory of evolution, is it just a theory? Well, it's about as strong as theories get. And without it, as Theodosia Dobzhansky said, biology as we know it, and all of the progress we've made in b... | Evidence for evolution Biology Khan Academy (2).mp3 |
And without it, as Theodosia Dobzhansky said, biology as we know it, and all of the progress we've made in biology, frankly, wouldn't make any sense and probably would not have happened. Now, I'm going to broadly go into three types of evidence in this video for evolution and natural selection. The first is structural.... | Evidence for evolution Biology Khan Academy (2).mp3 |
And these are the types of things that folks like Darwin would have observed, that people have been observing in biology for a long time, but evolution and natural selection starts to make a lot more sense of it. And here we're talking about the macro structure, things that we can, for the most part, observe with our e... | Evidence for evolution Biology Khan Academy (2).mp3 |
Microbiology, and especially in genetics. So this has really firmed up the theory of evolution. And then the last dimension we'll look at is direct observation. Direct observation. And this is really where it goes beyond a theory. We are seeing it happen. A lot of times people say, oh, it's a theory, it happened, the t... | Evidence for evolution Biology Khan Academy (2).mp3 |
Direct observation. And this is really where it goes beyond a theory. We are seeing it happen. A lot of times people say, oh, it's a theory, it happened, the theory says it happened over tens of millions of years, but no one was around to really observe, even if we see a lot of evidence, no one knows if it for sure hap... | Evidence for evolution Biology Khan Academy (2).mp3 |
A lot of times people say, oh, it's a theory, it happened, the theory says it happened over tens of millions of years, but no one was around to really observe, even if we see a lot of evidence, no one knows if it for sure happened. But if you're directly observing things, well, you know it's for sure happening. And as ... | Evidence for evolution Biology Khan Academy (2).mp3 |
So let's go through each of these. So first let's talk about structural. And this is a very high-level overview. I encourage you to do more research on it. You will find loads and loads and loads of any type of this evidence. So the first thing I wanna talk about is homologous structures, homologous structures that you... | Evidence for evolution Biology Khan Academy (2).mp3 |
I encourage you to do more research on it. You will find loads and loads and loads of any type of this evidence. So the first thing I wanna talk about is homologous structures, homologous structures that you see throughout the biological world. Homologous, homologous structures. And the word homologous means things tha... | Evidence for evolution Biology Khan Academy (2).mp3 |
Homologous, homologous structures. And the word homologous means things that have similar structures, similar position, similar ancestry, but not necessarily the exact same function. And here you see examples of a, well, as a human we would consider a forearm. You see the human forearm and wrist. And then you see the h... | Evidence for evolution Biology Khan Academy (2).mp3 |
You see the human forearm and wrist. And then you see the homologous structures in dogs and birds and whales. And even though this part of those animals have very different functions, a human does not walk on its hands for the most part. A dog does walk on its front legs. A bird isn't walking at all. It's using them to... | Evidence for evolution Biology Khan Academy (2).mp3 |
A dog does walk on its front legs. A bird isn't walking at all. It's using them to flap its wings. And a whale, this is making up its actual fins. It's using them to propel or to control their movement inside of the water. And even though they have these very, very different functions, and at first when you look at a h... | Evidence for evolution Biology Khan Academy (2).mp3 |
And a whale, this is making up its actual fins. It's using them to propel or to control their movement inside of the water. And even though they have these very, very different functions, and at first when you look at a human and a bird and a whale, on the outside they might look reasonably different. When you look at ... | Evidence for evolution Biology Khan Academy (2).mp3 |
When you look at these bone structures, they are eerily similar, especially with the color-coded, especially color-coded the way it is. So these are, this is a very strong hint that maybe humans, dogs, birds, and whales share a common ancestor more recently in the past than say other animals or organisms, I should say,... | Evidence for evolution Biology Khan Academy (2).mp3 |
Now these are all species that exist today. These are all species that exist on the planet at the same time. But we also see structural evidence by going into the fossil record. In the last few hundred years, or really in the last hundred years is where we've gotten really good at it, we've gotten good at looking at di... | Evidence for evolution Biology Khan Academy (2).mp3 |
In the last few hundred years, or really in the last hundred years is where we've gotten really good at it, we've gotten good at looking at different layers of different layers of rock strata and being able to date them and saying, okay, that layer was laid down X million years ago. That layer was laid down a little bi... | Evidence for evolution Biology Khan Academy (2).mp3 |
And then looking at fossils within that to say, okay, 20 million years ago, there were species around that looked like that. And then 10 million years ago, there were species that looked like that. And one example is if you look at horse-like animals. So this is right over here. We're talking about horses, zebras, donk... | Evidence for evolution Biology Khan Academy (2).mp3 |
So this is right over here. We're talking about horses, zebras, donkeys, mules, things like that. The modern ones, well, this is their bone structure. But if you look at the fossil record from 12 to five million years ago, you see fossils that look like this. And they're very close. So you see, it's very believable tha... | Evidence for evolution Biology Khan Academy (2).mp3 |
But if you look at the fossil record from 12 to five million years ago, you see fossils that look like this. And they're very close. So you see, it's very believable that you see you could have evolution from this to that. But then you go further back, and once again, it seems like a very gradual process. And once agai... | Evidence for evolution Biology Khan Academy (2).mp3 |
But then you go further back, and once again, it seems like a very gradual process. And once again, this is happening over, these are from 12 to five million years ago, these are from 16 to 12 million years ago, these are from over 34 million years ago. And so you can see how this is happening at a very, very gradual p... | Evidence for evolution Biology Khan Academy (2).mp3 |
And the mechanism, and we go into some depth in other videos in Khan Academy, you have variation species, you have the environment selecting for it. The environment might change, or different things happen, so you have different forms of selection, different types of combinations sprout up, they're more suitable for th... | Evidence for evolution Biology Khan Academy (2).mp3 |
And so when you look at the fossil record, it makes a lot of sense that, okay, this is strong evidence for evolution, that the animals that we see today weren't just put on, just created all of a sudden, and haven't changed since then, that there's a constant change, and we can see it directly through the fossil record... | Evidence for evolution Biology Khan Academy (2).mp3 |
And you can see at the early stages, they look eerily similar. In fact, you see proto-gill slits in all of these animals, which later differentiate into things that are more suitable for what that animal actually becomes. And Haeckel, he's the guy who coined ontogeny recapitulates phylogeny, which is a very fancy way o... | Evidence for evolution Biology Khan Academy (2).mp3 |
But his drawings and his observations, this is compelling evidence for life-sharing a common ancestry, coming from similar origins that got more and more different over time through the process of natural selection. So everything I've talked about so far has been kind of macro-structure, things we can observe. The next... | Evidence for evolution Biology Khan Academy (2).mp3 |
Micro-biology. And biolo-biology. Microbiology. And the more we understand about microbiology, the more compelling case of evolution. Because when we look at even, you know, one, all life forms that we know, they involve DNA. How the DNA gets replicated and translated and transcribed is very similar from one life form ... | Evidence for evolution Biology Khan Academy (2).mp3 |
And the more we understand about microbiology, the more compelling case of evolution. Because when we look at even, you know, one, all life forms that we know, they involve DNA. How the DNA gets replicated and translated and transcribed is very similar from one life form to another. The idea of DNA going to, DNA coding... | Evidence for evolution Biology Khan Academy (2).mp3 |
The idea of DNA going to, DNA coding for proteins, proteins that are made up of amino acids is something that we see throughout biology. Amino acids, which once again, hints at a common ancestry. And not only are those molecular, and many of the very proteins, are very, very similar, more similar than if you looked at ... | Evidence for evolution Biology Khan Academy (2).mp3 |
And not just do they share these common micro-structures and processes, but the actual information stored in things like DNA also are very, very strong evidence for evolution. So this is a picture, I got this from, I got this from the site, I should give proper credit, 23andme.com. But this, and you'll see other data l... | Evidence for evolution Biology Khan Academy (2).mp3 |
And these numbers tell us how much genetic similarity at a high level do we have with chimpanzees, mice, fruit flies, yeast, and plants. And the fact that we have 26% of our genes in common with yeast is mind-blowing. Because at a macro level, it doesn't seem like there's a lot in common with yeast. But when you get at... | Evidence for evolution Biology Khan Academy (2).mp3 |
But when you get at a microbiological level, there's a good bit that's in common with yeast. And chimpanzees, we do relate to them. Their facial expressions often feel eerily human, their behaviors often feel eerily human, but their genes show just how close to human beings they actually are. And this actually shows th... | Evidence for evolution Biology Khan Academy (2).mp3 |
And this actually shows that even, you know, mice are way closer, if you looked at the entire tree of life, based on genetic evidence, things like mice and even fruit flies are awfully close to human beings, especially if you were to compare it to bacteria or plants. But once again, you share all of these common proces... | Evidence for evolution Biology Khan Academy (2).mp3 |
Direct evidence of evolution. And I talk about this in the first evolution video. But the direct evidence we see all the time with things like bacteria, where you have bacteria, let's say growing around, and we have antibiotics that we use in our body to kill bacteria. But the reason why many physicians and scientists ... | Evidence for evolution Biology Khan Academy (2).mp3 |
But the reason why many physicians and scientists will tell you don't overuse antibiotics is because the more you use it, it causes a very strong natural selection process for bacteria that are going to be resistant to that antibiotic. So if you keep using an antibiotic and the bacteria keep changing, there's more and ... | Evidence for evolution Biology Khan Academy (2).mp3 |
So if you have a population of circles, obviously a very simple model here, maybe some of these circles are that off-white color, maybe some of them are blue, and maybe some of them are this salmon color. For certain traits, your environment might make certain of them better for reproduction, better for survival, evadi... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Well then, in the next generation, in the next generation, because the blue's more likely to be able to get to reproduction, because they weren't eaten, you're likely to have more blues. So let me draw a few more blues, and maybe a little bit less of the other ones, because they're also competing for resources amongst ... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
But as you can see, if in this environment, blue seems to carry some advantage, even if it's a slight probabilistic advantage, over many generations, blue will start to dominate. And so you start to see that evolution of this population to being more blue as a species. So you have these blue circles. So one way to thin... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
So one way to think about it is you have variation in a species is really what natural selection is based off of certain variants might be more favorable than others. So that is what's really necessary for natural selection to fuel evolution, to fuel evolution. Now, a key question is, where does this variation in a pop... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
And to think about that, we just have to remind ourselves where our phenotypes come from. How do these expressed traits get expressed? Well, in all the living organisms we're aware of, we have DNA. As human beings, we have 23 pair of chromosomes, and each chromosome you could view as just a very, very, very, very long ... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
As human beings, we have 23 pair of chromosomes, and each chromosome you could view as just a very, very, very, very long strand of DNA, and sections of that DNA code for various traits. And each of those sections that code for, say, a certain protein or a part of an enzyme, we call those things genes. We call those th... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
So we have multiple chromosomes. We have 20, as human beings, different species have different number, but as human beings, we have 23 pair of chromosomes. Each chromosome you view as a long strand of DNA. Parts of that DNA code for specific genes. And then if you were to zoom in, if you were to zoom in on those genes,... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Parts of that DNA code for specific genes. And then if you were to zoom in, if you were to zoom in on those genes, you would see these nucleotide sequences. This is all a review. We've seen this in other videos, where you see your adenine, your guanine, your cytosine, your thymine, in order that carries the information... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
We've seen this in other videos, where you see your adenine, your guanine, your cytosine, your thymine, in order that carries the information that will eventually be coded into mRNA, which then gets coded into protein. Now, there's two primary sources of variation. One source of variation is sexual reproduction. Sexual... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Sexual reproduction. Now, not all organisms reproduce sexually, but many of the ones that we know, including human beings, do. We're a male member of the species and a female member of the species. Each contribute a random half of their chromosomes to the next organism. So one way to think about sexual reproduction is ... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Each contribute a random half of their chromosomes to the next organism. So one way to think about sexual reproduction is it keeps shuffling the different versions of the genes that you have in the population into different combinations of those versions of genes. And so that generates variation. But sexual reproductio... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
But sexual reproduction by itself will not create new versions of genes, which we call alleles, or new genes entirely. And so the primary way that that happens is through mutations. And you might have guessed that we were going to talk about that, because I had this title up here. So another source of variation, and yo... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
So another source of variation, and you could almost view this as a more fundamental one, because this would happen even in organisms that aren't reproducing sexually, is that over time, there could be just random mistakes. There could be edits to these genes. And it could be a random, maybe this G gets turned into a C... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Or maybe this T and A gets cut out during the DNA replication process. These mutations, which are all about genotype, and let me make this very clear. So when we're looking at this sequence, we're thinking about genotype. Differences in genotype are not always obvious from expressed traits. So sometimes they do change ... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
Differences in genotype are not always obvious from expressed traits. So sometimes they do change phenotype, or they're observable in phenotype. Sometimes they're not. But when they are observable in phenotype, as I just mentioned, many times it could be a negative change in phenotype, where it makes it less viable for... | Mutation as a source of variation Gene expression and regulation AP Biology Khan Academy (2).mp3 |
And you can imagine there's no more obvious thing to classify than all of the living things around us, than all of the life that around us. So what I want to start talking about is how do we classify all of the life around us. And this is more often generally referred to taxonomy. But the most basic question you have w... | Species.mp3 |
But the most basic question you have when you look at all of the life around you, you start to see similarities between some of these living things. You see, obviously, this thing right over here is more similar to the things that look like it than it does to the grass behind it or to that tree. And so we start saying,... | Species.mp3 |
And that very most building block of how we classify all of the living things around us is putting them into buckets called species. So for example, this is one particular animal, but we see other animals that seem to look like it. And so we say they're all part of the species of lions. And this animal, it's one animal... | Species.mp3 |
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