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Natural selection in peppered moths

Natural selection leads to the predominance of certain traits in a population, and the suppression of others. Created by Sal Khan.

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Video transcript

- [Narrator] You might be familiar with the idea of evolution, that species change over time. And you can see that if you look at old bones, old fossils, how they change through the fossil record. But the obvious question is, how do these species actually do that? What is the mechanism? Well, one of the major mechanisms is natural selection. So natural selection is all based on at any given point in time, there is variation in a species. There are different traits that are expressed in different ways. And many times, a lot of those variations are fine, but then you can have environmental factors which make some of those traits more favorable than others. And if those traits are more favorable to be able to survive, to be able to get food, or to be able to reproduce, well then the genes that code for those traits are more likely to be passed on generation after generation. And to make this very tangible, I will give you the example of the peppered moth, which is probably the most common example when people show evidence of natural selection. If you were to go to the mid 1800s in London, most of the peppered moths were the white peppered moths. There were very few black peppered moths, but there was variation. There would be some lighter ones, there would be some darker ones, and in-between. Now what's interesting is between the mid 1800s and the late 1800s, you have the industrial revolution in London really hitting full gear. And so there was a lot of pollution in the air. And so a lot of the surfaces that the moths might rest on like trees, or the wall of a building, became darker and darker. So as where these moths could rest became darker and darker, what do you think might have happened? Well, some of you might guess. In this different environment now, as the environment has gradually changed, all of a sudden having the traits that make you darker will be more favorable than they were even 30 or 40 or 50 years ago. And the white trait, which might've been okay in the mid 1800s, now all of a sudden makes these moths very obvious to see. So if there was a bird that was looking for lunch, it'd be very easy to pick off the white moths versus the black moths. And what we saw is actually by the 1900s, most of the peppered moths in London were now darker in color. And so what you had happening generation after generation is variation in the moth color. But as the background environment became darker, the ones that were white colored were easier to pick off by predators. And if they're picked off by a predator, they're definitely not reproducing and passing on their genes. And then the ones that were able to survive and pass on their genes were the darker moths. Now it's been interesting over the last 50 or so years as environmental regulations have gone into effect, and the air has started to clean up in places like London, you're seeing a return of the white peppered moth. Because once again, the surfaces are no longer covered with soot, and so the variants that are lighter in color now have a decent chance of not being spotted. And if anything, now the darker ones might have a better chance of being spotted in this cleaner environment.