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Genetic drift, bottleneck effect, and founder effect

AP Bio: EVO‑1 (EU), EVO‑1.H (LO), EVO‑1.I (LO)

Video transcript

- [Voiceover] We've already made several videos over evolution, and just to remind ourselves what evolution is talking about, it's the change in heritable traits of a population over generations. And a lot of times, you'll hear people say evolution and Natural Selection really in the same breath, but what we wanna make a little bit clear in this video is that Natural Selection is one mechanism of evolution. It's the one most talked about because it is viewed as the primary mechanism. Natural Selection. But what we're gonna talk about in this video is another mechanism called Genetic Drift. So there's Natural Selection, and there is Genetic Drift. Now we've done many videos on Natural Selection, but it's this idea that you have variation in a population, you have different heritable traits, and I'm gonna depict those with different colors here. We have a population of living circles here, (laughs) and they could come in blue or maybe magenta. Maybe they come in another variation too, maybe there is yellow circles, and Natural Selection is all about which of these traits are most fit for the environment so that they can reproduce. So there might be something about being, say, blue, that allows those circles to reproduce faster, or to be less likely to be caught by predators, or to be able to stalk prey better. Even if they're only slightly more likely to reproduce, over time, over many generations, their numbers will increase and dominate, and the other numbers are less likely, or the other trait is less likely to survive, and so we will have this Natural Selection for that blue trait. So this is all about traits being the fittest traits. Now Genetic Drift is also change in heritable traits of a population over generations, but it's not about the traits that are most fit for an environment are the ones that necessarily survive. Genetic Drift is really about random. Random changes. Random changes, and a good example of that I have right over here that we got from, I'll give proper credit, this is from OpenStax College Biology, and this shows how Genetic Drift could happen. So right over here, I'm showing a very small population of 10 rabbits, and we have the gene for color, and we have two versions of that gene, or we could call them two alleles. You have the capital B version, and you have the lower case B, and capital B is dominant. This is kind of a very Mendelian example that we're showing here. And so if you have two of lower case genes, two of the white alleles, you're going to be white. If you have two of the brown alleles, the capital Bs, you're going to be brown, and if you're a heterozygote, you're still going to be brown. So as you can see here, there are several heterozygotes in this fairly small population. But if you just count the capital Bs versus the lower case Bs, you see that we have an equal amount of each. And so the frequency, if you were to pick a random allele from this population, you're just as likely to pick a capital B than a lower case B. Even though the phenotype, you see a lot more brown, but these six brown here have both the upper case B and the lower case B. Now let's say they're in a population where whether you are brown or whether you are white, it confers no advantage. There's no more likelihood of surviving and reproducing if you're brown than white, but just by chance, by pure random chance, the five bunnies on the top are the ones that are able to reproduce, and the five bunnies on the bottom are not the ones that are able to reproduce. And you might be saying hey, why did I pick those top five? I didn't pick them, I'm just giving an example. It could've been the bottom five. It could've been only these two, or the only two white ones were the ones that were able to reproduce. It's by pure random chance, or it could be because of traits that are unrelated to the alleles that we are talking about. But from the point of view of these alleles, it looks like random chance. And so in the next generation, those five rabbits reproduce and you could have a situation like this, and just by random chance, as you can see, the capital B allele frequency has increased from 50% of the alleles in the population to 70%. And then it could be another random chance, and I'm not saying this is necessarily going to happen. It could happen the other way. It could happen even though that first randomness happened, maybe now all of a sudden this white rabbit is able to reproduce a lot, but maybe not. Maybe these two brown rabbits that are homozygous for the dominant trait are able to reproduce, and one again it has nothing to do with fitness. And so they're able to reproduce, and then all of a sudden, the white allele is completely gone from the environment. And the reason why this happened isn't because the white allele somehow makes the bunnies less fit. In fact, it might have even conferred a little bit of an advantage. It might have been, from the environment that the bunnies are in point of view, it might have even been a better trait, but because of random chance, it disappears from the population. And the general idea with the Genetic Drift, so once again, just to compare, Natural Selection, you are selecting, or the environment is selecting traits that are more favorable for reproduction, while Genetic Drift is random changes. Random changes in reproduction of the population. Now, as you can imagine, I just gave an example with 10 bunnies, and what I just described is much more likely to happen with small populations. So much more likely. More likely with small populations. And we have videos on statistics on Khan Academy, but the likelihood of this happening with 10 bunnies versus the likelihood of what I just described happening with 10 million bunnies is very different. It's much more likely to happen with a small population. So a lot of the contexts of Genetic Drift are when people talk about small populations. In fact, many times Biologists are worried about small populations specifically because of Genetic Drift. For random reasons, you could have less diversity, less variation in your population, and even favorable traits could be selected for by random chance. There's two types of Genetic Drift that are often called out that cause extreme reductions in population, and significantly reduce the populations. One is called the Bottleneck Effect. Let me write this down. So the Bottle, Bottleneck, the Bottleneck Effect, and then the other is called the Founder Effect. Do that over here. The Founder, Founder Effect. They are both ideas where you have significant reduction in population for slightly different reasons. Bottleneck Effect is you have some major disaster or event that kills off a lot of the population, so only a little bit of the population is able to survive. And the reason why it's called Bottleneck is imagine if you had a bottle here. If you had a bottle here and, I dunno, inside of that bottle, you had marbles of different colors. So you have some yellow marbles, you have some magenta marbles, you have some, I don't know, blue marbles. These are the colors that I tend to be using. You have some blue marbles, so you have a lot of variation in your original population. But if you think about pouring them out of a bottle, maybe somehow there's some major disaster, and only two of these survive, or let's say only four of these survive, and so you could view that as, "Well, what are the marbles that are getting poured "out of the bottle?" It's really just a metaphor. Obviously, we're not putting populations of things in bottles. But after that disaster, only a handful survive, and they might not have any traits that are in any way more desirable or more fit for the environment than everything else, but they just by random chance, because of this disaster, they are the ones that survived. And so all of a sudden, you have a massive reduction not only in the population, but also in the variation in that population, and many alleles might have even disappeared, and so you have an extreme form of Genetic Drift actually occurring. Another example is Founder Effect, which is the same idea of a population becoming very small, but the Founder Effect isn't because of a natural disaster. Let's say you had a population. Once again, you have a lot of different alleles in that population. You have a lot of variation, you have a lot of variation in that population. So let me just keep coloring it. You have a lot of variation in this population, and let's say that, you know, they're all hanging out in their region, and maybe, you know, they are surrounded by mountains. I'm just making this up as I go, but let's say a couple of these blue characters were out walking one day, and they maybe get separated from the rest of their population. Maybe they discover a little undiscovered mountain pass, and they go settle a new population someplace. So that's why it's called the Founder Effect. These are the founders of a new population, and once again, by random chance, they just have a lot less variation. They're a smaller population and they happen to be disproportionately or all blue in this case, and so now this population is going to (mumbles) Just the process of this was Genetic Drift where many alleles will have disappeared because you have such a small population of blues here. And also because you have such a small population, you're likely to have even more Genetic Drift. So it's a really interesting thing to think about. Evolution and Natural Selection are often talked about hand in hand, but Natural Selection isn't the only mechanism of Evolution. You also have Genetic Drift, which is really about, not selecting for favorable traits, it is about randomness.