Genetic drift, bottleneck effect, and founder effect

we've already made several videos over evolution and just remind ourselves what evolution is talking about 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 want to make a little bit clearer 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 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 - maybe there's 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 and so they are even they're only slightly more likely to reproduce over time over many generations their numbers will their numbers will increase and dominate and the other numbers are more are less likely are less likely to 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 a out random 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 the 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 we have a population of 10 rabbits and we have the gene for four-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 lowercase B and capital B is dominant this is we're just kind of a very Mendelian example that we're showing here and so if you have two lowercase if you have two of the lowercase genes two of the white 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 and if you're a heterozygote you're still going to be brown so as you can 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 PS 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 then a lowercase B even though the phenotype you see a lot more Brown but these these six brown here have both the uppercase B and lowercase B now let's say they're in a population where they where whether you're 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 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 why 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 and so in the next generation 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 saying this is necessarily going to happen it could happen the other way it could happen even though that that first that first randomness happened maybe now sudden this white rabbit is able to reproduce a lot but maybe not maybe two of maybe these two brown rabbits that are homozygous for the dominant trait are able to reproduce and once again has nothing to do with Fitness and so they're able to reproduce and all of a sudden the white allele has been is completely gone from the environment and the reason why this happened isn't because the white allele is somehow makes the bunnies less fit in fact it might have even conferred a little bit of an advantage it might have been a permanent from the environment that the bunnies are in point of view it might have been a better trait but because of random chance it it disappears from the population and the general idea with the genetic drift so once again just to compare natural selection you're selecting trait 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 ten bunnies and what I just described is much more likely to happen with small populations so much more likely more likely with with small populations if if I and we have videos on statistics on Khan Academy but the likelihood of this happening with ten bunnies versus the likelihood of what we had 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 or when people talk about small populations in fact many times mile just are worried about small populations specifically because of a drift for random reasons you could have less diversity less variation in your population and even favorable treats favorable traits could be selected for by random by random chance now there's particular there's two tip 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 bottle neck the bottleneck effect and then the other is called the founder effect do that over here the founder founder effect and they are both they are both ideas where you have significant reduction in population for slightly different reasons bottleneck the 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 because 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 he has 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 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 is well what are the marbles that are getting poured out of the bottle it's kind of just 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 they might not have any traits that are 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 the population but also in the very in the variation in that population and many deels might have even disappeared and so you have you have an extreme form of genetic drift actually occurring another example is founder effect 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 it's 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 and in in their region and maybe you know they are they are surrounded by they're surrounded by mountains and I'm just making this up as I go but let's say a couple of these blue characters we're 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 of fact 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 have they happen to be disproportionately or all blue in this case and so now this population is going to one a few you might have already had this is just the process of this with genetic drift where you have so many alleles will have disappeared because you have such a small population of blues here and also because you have 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