Genetic drift, bottleneck effect and founder effect | Biology | Khan Academy

We've already made several videos over evolution. Just to remind ourselves what evolution is talking about: it's the change in heritable traits of a population over generations. A lot of times, you'll hear people say "evolution" and "natural selection" really in the same breath. But what we want to make a little bit clear in this video is that natural selection is one mechanism of evolution, and it's the one most talked about because it is viewed as the primary mechanism.

Natural selection, but we're going to 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. I'm going to depict those with different colors here. We have a population of living circles here, and they could come in blue or maybe magenta. Maybe they come in another variation too; maybe there are yellow circles.

Natural selection is all about which of these traits are most fit for the environment so that they can reproduce. 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. The other trait is less likely to survive, and so we will have this natural selection for that blue trait. This is all about traits being the fittest traits.

Now, genetic drift is also a change in heritable traits of a population over generations, but it's not about the traits that are most fit for an environment being the ones that necessarily survive. Genetic drift is really about random changes. 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. We have a population of 10 rabbits, and we have the gene for color. We have two versions of that gene, or we could call them two alleles. You have the capital B version, and you have the lowercase b. Capital B is dominant—this is where we're just kind of a very Delian example that we're showing here.

If you have two lowercase alleles, you're going to be white. If you have two of the brown alleles, the capital B's, you're going to be brown. If you're a heterozygote, you're still going to be brown. As you can see here, there are several heterozygotes in this fairly small population. But if you just count the capital B's versus the lowercase b's, you see that we have an equal amount of each. If you were to pick a random allele from this population, you're just as likely to pick a capital B as a lowercase b, even though the phenotype you see is a lot more brown.

These six brown here have both the uppercase B and the lowercase b. Now, let’s say they're in a population where whether you're brown or white 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.

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 have been the bottom five. It could have 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.

In the next generation, those five rabbits reproduce, and you could have a situation like this. 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%. Then it could be another random chance. 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 two of these two brown rabbits that are homozygous for the dominant trait are able to reproduce. Once again, it has nothing to do with fitness, and so they’re able to reproduce. All of a sudden, the white allele has been completely gone from the environment. 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.

From the environment that the bunnies are in point of view, it might have been a better trait. But because of random chance, it disappears from the population. The general idea with genetic drift is, once again, just to compare natural selection: you're selecting traits, or the environment is selecting traits that are more favorable for reproduction, while genetic drift is random changes in reproduction of the population.

Now, as you can imagine, I just gave an example with 10 bunnies. What I just described is much more likely to happen with small populations. 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. A lot of the context of genetic drift or 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. Even favorable traits could be selected for by random chance. Now, there are two types of genetic drift that are often called out that cause extreme reductions in population and significantly reduce the population. One is called the bottleneck effect. Let me write this down: the bottleneck effect. The other is called the founder effect.

Do that over here: the founder effect. They are both ideas where you have significant reduction in population for slightly different reasons. The bottleneck effect is when 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. The reason why it’s called bottleneck is imagine if you had a bottle here. If you had a bottle here, and I don’t know, 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.

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 just by random chance, because of this disaster, they are the ones that survived.

All of a sudden, you have a massive reduction not only in the population but also in the variation in that population. Many alleles might have even disappeared, and so you have an extreme form of genetic drift actually occurring. Another example is the 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 in that population, so let me just keep coloring it. You have a lot of variation in this population. Let’s say that they’re all hanging out in their region, and maybe 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 to settle a new population someplace. That’s why it’s called the founder effect. These are the founders of a new population. Once again, by random chance, they just have a lot less variation. They’re a smaller population, and they happen to be disproportionately—and they’re all blue in this case.

Now, this population is going to, you might have already had this, just the process of this was genetic drift, where so many alleles will have disappeared because you have such a small population of blues here. Also, because you have a small population, you’re likely to have even more genetic drift. 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.