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Video transcript

- [Voiceover] 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 started with this quote "Nothing in biology makes sense except in the light of evolution." This is by Theodus Dobzhansky, who's a famous biologist, he's passed away now, 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 about. And since Darwin had come up with this theory in the mid-19th century, we've had far more tools to back it up beyond just the observations we had up until that point. We have our tools around dating and the fossil record, which gives us much more evidence. We have our tools of microbiology and genetics, which gives us even stronger evidence, so a lot of times people say "Oh, it's a theory, evolution "is it just a theory?" Well, it's about as strong as theories get and without it, as Theodus 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. 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 eyes or with a very simple microscope. The next level is what we've learned, really over the last 100 or so years, at the micro level, in microbiology. 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, "you know, 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 we'll see, evolution does not only occur over time scales of millions or tens of millions of years, it actually can occur and we see it occurring all the time on scales well within a human observational capacity, within just a matter of months or years. 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 want to talk about is homologous structures, homologous structures, that you see throughout the biological world. Hom-ol-o-gous, homologous structures. And the word homologous means things that have similar structure, 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 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 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 human and a bird and a whale on the outside, they might look reasonably different, when you look at these bone structures, they are eerily similar, 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, that don't have, whose structures aren't as homologous as these are right over here. And if you were independently trying to create structures for what each of these different species are doing, it's not obvious that you would have such homologous structures actually be involved. 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 early in the last hundred years is where we've gotten really good at it, we've gotten good at looking at different layers of rock strata and being able to date them and say "Okay, that layer was laid down x-million "years ago, that layer was laid down a little bit "more recent, this one was even more recent," and then looking at fossils within that to say "Okay, 20 million years ago there were species "around that looked something like that, "and then 10 million years ago there were species "that looked like that." And one example is if you look at a horse-like animal. 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 5 million years ago, you see fossils that look like this and they're very close so you see, it's very believable that you could have evolution from this to that, but then you go further back and once again, it draws, it seems like a very gradual process. And once again, this happening over, these are from 12 to 5 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 pace and the mechanism, and we go into some depth in other videos in Khan Academy, you have variation in 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 the environment, they start to reproduce better, they become the dominant species or they take over certain parts of a niche or an ecosystem, and so you have this change, this heritable change of traits over time. 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, but there's a constant change and we can see it directly through the fossil record. Now the next point of evidence, I will put a bit of a caveat because the gentleman who first created this, his name was Haeckel, he was a controversial figure, he had some spurious theories, and even this diagram that he created, it seems like he fudged a little bit of the drawings in order to make a stronger argument, but even with modern observations, these drawings are pretty close to being correct. And it's very, very compelling. It shows the embryonic development of a whole series of species, from a fish on the left to a reptile to birds to mammals and another mammal, to non-human mammals and, of course, to humans. 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 claimed ontogeny recapitulates phylogeny, which is a very fancy way of saying that your embryonic development is telling the story of the evolutionary path, which isn't true, but you'll even hear people quote that today. 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 thing I'm gonna talk about is, you can think about as micro structures or processes, and this is microbiology. Microbiology... Biolo, biology... Microbiology. And the more we understand about microbiology the more compelling case of evolution because when we look at even 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 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 look at the macro level or even at the structural level between different species, and not just do they share these common microstructures 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 like this that's very similar to this, which is how much genetic similarity do we have between different species, 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 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, so just how close to human beings they actually are. And this actually shows that even mice are way closer, if you look 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 a plant. But once again, you share all of these common processes and the fact that we can now measure how far things are away allows us to create a very accurate tree of life, and especially thinking about how far in the past we had evolutionary common ancestors. Now the last thing that I promised I would talk about is direct evidence, 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 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 bacterias keep changing, there's more and more variation, well you're gonna kill a lot of the bacteria but if even one of them is resistant to that antibiotic that you use, well then all of its competition is gonna get killed and so that drug-resistant superbug, it's often called, is going to be able to go nuts and that antibiotic isn't going to be able to do anything. And if you look at science today or you look at medicine today, this is kind of an arms race. You have this constant need to create new antibiotics because more and more bacteria are becoming drug-resistant, they're becoming what's often called superbugs, where they are resistant to the existing antibiotics. And this is evolution, natural selection, happening on a human scale. You could also think about the flu virus where every year that vaccine for the flu virus, you gotta get a new one every year because the virus is changing. Your immune system's ability to recognize it can't recognize the next year's because it's changed so much.
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