Evidence for evolution | Common ancestry and phylogeny | High school biology | Khan Academy

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. I starting with this quote: "Nothing in biology makes sense except in light of evolution." This is by Theodosius Dobzhansky, who's a famous biologist. He's passed away now, and what he's saying is absolutely true.

This is why it's so important to appreciate the evidence for evolution and natural selection and to understand them. Before the theory of evolution, biology was just about observation and classification without having a cohesive narrative for how all of this came about. Since Darwin came 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 in the fossil record, which gives us much more evidence. We have our tools of microbiology and genetics, which give us even stronger evidence. A lot of times people say, “Oh, it's a theory. Evolution is just a theory.” Well, it's about as strong as theories get. Without it, as Theodosius 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. These are things people have been observing in biology for a long time, but evolution and natural selection start to make a lot more sense of it. 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 hundred years at the micro level in microbiology and especially in genetics. This has really firmed up the theory of evolution. The last dimension we'll look at is direct observation. 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 happened. But if you're directly observing things, well, you know it's for sure happening.

As we see, evolution does not only occur over time scales of millions or tens of millions of years. It can actually 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.

First, let's talk about structural. This is a very high-level overview. I encourage you to do more research on it; you will find loads and loads of any type of this evidence. The first thing I want to talk about is homologous structures—homologous structures that you see throughout the biological world. The word homologous means things that have similar structures, similar position, similar ancestry, but not necessarily the exact same function.

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 structure in dogs, birds, and whales. Even though this part of those animals has 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 is using them to propel or to control its movement inside of the water.

Even though they have these 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 with the way it is color-coded. 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 that don't have structures as homologous as these.

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 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 recently. This one was even more recent.”

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.” One example is if you look at a horse-like animal, so this is talking about horses, zebras, donkeys, mules, things like that—the modern ones—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. You feel it's very believable that you could have evolution from this to that. Then you go further back, and once again it seems like a very gradual process.

This is happening over, you know, 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 you can see how this is happening at a very gradual pace. The mechanism—and we go into some depth in other videos on 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. You have this heritable change of traits over time.

When you look at the fossil record, it makes a lot of sense that this is strong evidence for evolution—that the animals we see today weren't just created all of a sudden and haven't changed since then. There is 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. Even this diagram that he created 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 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. 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.

Haeckel is the guy who coined the phrase, “ontogeny recapitulates phylogeny,” which is a very fancy way of saying that your embryonic development is telling the story of the evolutionary past. This isn't true, but you'll even hear people quote that today. However, his drawings and his observations are 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 going to talk about is what you can think of as micro-structures or processes, and this is microbiology. The more we understand about microbiology, the more compelling the case for evolution becomes.

When we look at all life forms that we know, they involve DNA. How the DNA gets replicated, translated, and transcribed is very similar from one life form to another. The idea of DNA going to DNA coding for proteins that are made up of amino acids is something we see throughout biology—amino acids, which once again hints at a common ancestry.

Not only are those molecular and many of the very proteins very, very similar, but they are more similar than if you looked at the macro level or even at the structural level between different species. Not just do they share these common micro-structures and processes, but the actual information stored in things like DNA also provides very strong evidence for evolution.

This is a picture I got from the site; I should give proper credit to 23andme.com. You'll see other data like this that's very similar to this, which is how much genetic similarity we have between different species. These numbers tell us how much genetic similarity, at a high level, we have with chimpanzees, mice, fruit flies, yeast, and plants.

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. Chimpanzees—yes, we 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.

This actually shows that even mice are way closer, if you looked at the entire tree of life based on genetic evidence. Mice and even fruit flies are awfully close to human beings, especially if you were to compare them to bacteria or plants. 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, 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 of evolution. 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 growing around and we have antibiotics that we use in our body to kill bacteria.

The reason why many physicians and scientists will tell you not to overuse antibiotics is because the more you use them, it causes a very strong natural selection process for bacteria that are going to be resistant to that antibiotic. If you keep using an antibiotic and the bacteria keep changing, there's more and more variation. You are going to kill a lot of the bacteria, but if even one of them is resistant to that antibiotic you use, then all of its competition is going to get killed.

That drug-resistant superbug—it's often called—will be able to thrive, and that antibiotic isn't going to be able to do anything. 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 are becoming what's often called superbugs, where they are resistant to the existing antibiotics.

This is evolution and natural selection happening on a human scale. You could also think about the flu virus, where every year, you know, that vaccine for the flu virus—you have to get a new one every year because the virus is changing. Your immune system's ability to recognize it cannot recognize the next year's because it's changed so much.