Genetic Drift - Online Tutor, Practice Problems & Exam Prep

Genetic drift is a change in allele frequency due to chance, particularly pronounced in small populations. It can lead to the loss of genetic variation as alleles may go to fixation or be lost unpredictably. Special cases like the founder effect occur when a small group establishes a new population, while a population bottleneck happens when a large population suddenly decreases in size, accelerating genetic drift. Both scenarios highlight that even large populations can exhibit low genetic variation if they experience significant reductions in size.

Genetic Drift

We've seen how natural selection can have very predictable effects on populations based on the fitness of the alleles. Well, if natural selection is predictable, genetic drift is not. And that's because we're going to define genetic drift here as a change in allele frequency due to chance. And this happens because population size in real populations is not infinite. Right?

Everything we've been talking about is based on probability in some way. Right? Hardy-Weinberg is based on probability. We defined fitness as the likelihood that an allele improves the survival or reproduction of an organism. That likelihood is a probability.

Now, if the population isn't infinite, that probability, that expectation and reality usually don't match. And especially, it's going to be more pronounced in small populations. And that difference between expectation and reality, that ends up being genetic drift. Alright. So let's see what we mean here.

We have these graphs for different population sizes, and in each one we're graphing on the y axis the frequency of the big A allele, in other words, p. And on the x axis, we're going to have generation time. And for all of these, we're going to start with the big A allele at 0.5, or 50% of the alleles will be big A in this population. And we're just going to see what happens if only genetic drift is acting on these alleles. Alright.

So let's imagine what's going to happen in this small population. Well, we're saying it's going to change due to chance. So what do we predict is going to happen to that big A allele? Well, it's just kind of bouncing around there, right, in very unpredictable ways. In every generation, it's changing some.

And because it's a small population, it's actually changing a decent amount every generation in ways, again, that you just can't predict. And in this case, that allele frequency went to 0. That big A allele was lost from the population. But what would happen if we, you know, sort of replayed this, took the same population, started with big A of 0.5 again and replayed it? Unpredictable.

Still going to bounce around a lot because we're in a small population, but where that allele frequency ends, we just can't say. Right? We'll play it out a few more times here. Again, here it's jumping around here. Well, that big A went to fixation.

It's the only allele left in the population. Do it 2 more times. Again, these random changes, those changes every generation are relatively large because it's a small population. But what they're going to be, completely unpredictable. Alright.

Now with that in mind, let's think what's going to happen in a medium-sized population. Well, we'll play this out again. I'll play out, sort of, all 5 populations at the same time because I think we understand what's going on here. But a larger population, we are still going to get those random changes, but every generation that changes just going to be a little less because a larger population probability is predicting what's going to happen a little bit better. There's a little less variation from that prediction, and it just doesn't change as much over time.

Now eventually, you can expect that these are going to bounce around enough that some of these alleles will be lost. Some will go to fixation. But again, they'll take longer, and it's unpredictable what they'll do. Now, for a large population, I bet you can think what's going to happen. Right?

We're still bouncing around. These populations are still becoming different from each other in unpredictable ways, but they're just not changing very much every generation because it's a really large population. Alright. So that means we can fill in this purple box here then. What is the effect on allele frequency?

Well, it's going to be random. Right? We cannot predict it, but we do want to note that this does reduce genetic variation because alleles are lost from the population. Right? We can see again in that small population, sometimes p went to 1, sometimes it went to 0, meaning that there was only one allele left in the population.

And these larger populations, that will happen as well. It's just going to take longer. Alright. We want to note down here that genetic drift is going to have the greatest effect on neutral alleles. Remember, we defined neutral alleles or neutral mutations as those that don't affect fitness.

They don't change the fitness of an organism. Well, if there's no fitness difference between the 2 alleles in the population, then genetic drift is really the main way that those allele frequencies are going to change. There's no fitness difference, so any change is just going to sort of be random. We do want to note though that it can affect alleles with fitness differences as well. Well, right here, it can even increase, so right with an up arrow there, the frequency of deleterious alleles in small populations.

Right? So even if an allele is not good, if it's bad, it reduces the fitness. If these random effects are big enough because it's a small population, that sort of random change might be bigger than the effect that natural selection is having, and it could increase the frequency of deleterious alleles or decrease the frequency of beneficial alleles. Alright. We're going to take a look at genetic drift a little bit more.

We're going to look at some specific types of genetic drift; for that, though, we have example and practice problems. Check them out.

Genetic Drift Example 1

This example tells us that the table below gives the allele frequencies for the big m allele in four different populations of sunflowers over 30 years. It says other than infinite population size, assume all other conditions of the Hardy-Weinberg principle are met. Then it asks, based on these data, which population do you expect to be the smallest? We have these four populations: A, B, C, and D.

We can see their allele frequencies for the big m allele here in 1990 and again 30 years later in 2020. Before we really look at the data, note that it's signaling that the condition of infinite population size is not met. So, think: what mechanism of evolution is introduced in non-infinite population sizes?

Non-infinite population sizes are affected by genetic drift, which is the random change in allele frequency. The smaller the population size, the greater the effect of genetic drift.

When examining these numbers, I search for the population with the greatest amount of change. That appears to be population C. Population C starts at an allele frequency of 0.5 and increases to an allele frequency of 1. In other words, the big m allele goes to fixation; it becomes the only allele in the population. That's the greatest amount of change, indicating that population C might be the smallest due to the significant impact of genetic drift. Therefore, I am going with C for my answer, attributing the greatest change to the significant effects expected in smaller populations due to genetic drift.

More practice after this. Give it a try.

Genetic drift will have the greatest impact on which type of alleles?

• A. Advantageous alleles.
• B. Neutral alleles.
• C. Deleterious alleles.
• D. Genetic drift impacts all alleles equally; it is population size that determines the strength of genetic drift.

Which of the following statements about genetic drift are true?

I) Genetic drift is most pronounced in large populations.

II) Genetic drift can reduce genetic variation.

III) Genetic drift is due to chance events, not differences in fitness.

• A. I & II only.
• B. I & III only.
• C. II & III only.
• D. I, II, & III.

Special Cases of Genetic Drift

Sometimes there may be a population that's very large, but it has surprisingly little genetic variation. Now remember, genetic drift can cause the loss of genetic variation as alleles are lost through just random change, but genetic drift has its greatest effect on small populations. So here we're going to look at special cases of genetic drift, and specifically, two particular situations that can result in accelerated genetic drift even in populations that end up being large. Alright. We're going to look at the founder effect and population bottleneck.

We'll start by looking at the founder effect here. In the founder effect, we're going to say a small subset of a population forms or founds a new population. Alright. So when that population is founded by just a few new individuals, well, that new population has allele frequencies matching the founders. Right?

So in our example here, we have this field of flowers, and you can see there are pink, white, and red flowers there. And we have allele frequencies, p equals 0.5, q equals 0.5 are 2 alleles here that we're looking at. There's an equal number in this population. But we have a barrier here, and it looks like just a couple of seeds were able to cross that barrier, and some new flowers are founding this new population. But just by chance, they're red flowers.

And our new allele frequency in this new population, well p equals 1. There's only one allele that made it across to this new population. In other words, that one allele has gone to fixation in this new population. And the other allele, well q equals 0, that other allele has been lost. Well, as these flowers, they're going to reproduce, and they're going to, you know, spread out, and that population might get very big in this new environment.

But the genetic variation, it's been lost already. Right? We've had this massive change in allele frequency because that population got very small. Those very that very small founding group didn't carry a lot of genetic variation with it. Alright.

So our other example here, kind of a related example, is population bottleneck. And a population bottleneck is when a sudden change to the environment reduces population size that later rebounds. Alright. So if you have a big population size, if that population crashes and gets very small for a little while and then gets big again, well, that's a population bottleneck. And when it was small, you're going to have that accelerated genetic drift because genetic drift happens the most in small populations.

So in that case, genetic drift is accelerated even though population size, again, is going to be large in the end. Alright. Now in this example, well, we have a field of flowers. Alright. The same field of flowers, same allele frequencies, p equals 0.5, q equals 0.5.

But here, well, we're showing in this case a fire. It could be different things that cause this, but in this case, a fire comes through, burns up, kills most of the plants except for just a few random ones there. And then those rare survivors, they reproduce. They sort of, you know, fill in the environment. They repopulate, but now our allele frequency has changed drastically.

Now p equals 0.18, q equals 0.82. You can imagine, you know, just as we looked at in founder effect, how alleles could be lost in this situation. And again, it was just a random effect by which organisms survived this environmental effect when the population got very small. Alright, so for both of these, the thing to take away, right, populations may be big, but what's most important for genetic drift is how small did they get at some point in time? Alright.

We'll look at this more and examples and practice to come. It'll be fun. I'll see you there.

Genetic Drift Example 2

In our example, we have Western Snowy Plovers, shorebirds that live along the American West Coast. Following a flood, a particular population of Western Snowy Plovers went from 437 down to just 26. Notably, Western Snowy Plovers are known for having large black spots on their faces, but the population that survived the flood had comparatively smaller facial spots. Assuming that face spot size is a heritable trait, let's answer the following questions:

A: What changes, if any, do you expect in the offspring of Western Snowy Plovers that survived compared to the original population? Given that the trait of facial spots can be inherited and considering the survivors exhibit smaller facial spots, it is expected that future generations will have smaller facial spots compared to the original population since these are the individuals that survived.

B: What special case of genetic drift describes this scenario? This scenario is indicative of a population bottleneck. This occurs when a population sharply reduces in size due to environmental events, which leads to a loss of genetic variation randomly among the survivors.

C: What are two ways genetic diversity could be increased in this population in the future? One method to restore genetic diversity could be through mutation, which can introduce new alleles and thus increase genetic variation over time. Another method could be gene flow, where alleles are brought in from other populations. If there is gene flow between this population and another Western Snowy Plover population that did not go through the population bottleneck, or one that preserved different alleles, genetic variation could be somewhat restored.

More practice after this. Give it a try.

True or False: if false, choose the answer that best corrects the statement.

The founder effect and population bottlenecks will generally decrease genetic variation in a population even if the population eventually returns to a large population size.

• A. True.
• B. False; the founder effect and population bottlenecks will only decrease the genetic variability if there is no natural selection.
• C. False; the founder effect and population bottlenecks will only reduce genetic variation if the population size remains small.
• D. False; population bottlenecks will decrease genetic variability, but the founder effect will introduce new genetic variation.

Humans first evolved in sub-Saharan Africa. Populations of modern humans likely migrated from Africa, populating the rest of the world within the last 100,000 years. A general trend observed in the human population is that the farther a native population is from Africa, the less genetic diversity in that population. What could explain this trend?

• A. Exchange of alleles through gene flow.
• B. Loss of alleles through a population bottleneck.
• C. Balancing selection and heterozygote advantage.
• D. Loss of genetic diversity through repeated founder effects.