Genetic drift is another way that we can get changes in allele frequencies, and it's due to what's called random sampling. Basically, the idea is that some alleles get lucky in terms of the numbers. So when we talked about Mendelian genetics, we looked at the probabilities of allele distributions from parents to offspring, and we saw that probability dictates that certain alleles will be distributed at certain frequencies. However, that doesn't always happen. Just because there is a probability that it will occur, doesn't necessarily mean that it will occur, and that's where randomness comes into play. You can see here that we have a nice 50-50 mix of blue and red marbles, but over the generations, our red marbles are more or less disappearing due to random sampling. And that is how genetic drift works. Basically, some alleles get lucky and others get unlucky, and we have a shift in allele frequencies because our allele distributions are not following those probability rules.

Now, the founder effect is when a small group from the population splinters off and forms a new one, taking a random sample of alleles with it. You see here in our starting population, there's a relatively even number of green and red dots. But our founding population doesn't have that same distribution. In fact, over time, we see that our founding population winds up with different allele and genotype frequencies than our original population, and that's due to the founder effect. The fact that these individuals all left the group, taking a random sample of alleles that was not necessarily reflective of this larger population's allele frequencies, means their new population will show different allele frequencies.

Now, the last type of genetic variation we are going to talk about is the bottleneck effect. This is when the population size dramatically decreases due to some random event, and this drastically alters allele frequencies. So, let me take myself out of the image here. You can see a nice chart of population size over time, and right here, that's where we have this bottleneck event, which causes our population to plummet. Either it's going to go extinct or recover and, assuming it recovers, well, when it recovers, it might wind up with a population of equal size and totally different allele colors—red, blue, green, and pink—but, our bottlenecking event leaves us with mostly red, a little blue, and a little green. We don't have any more pink or yellow in the mix anymore, and that means if our population recovers, the allele frequencies are going to look quite different than the initial population.

That's all I have for this video on genetic variation and selection. I'll see you guys next time.