Bozeman science: genetic drift

hi it's mr. Andersen and welcome to biology essentials video number three this is on genetic drift or random evolutionary change what I got to talk about in here is how we can get changes in a gene pool and so this right here represents a gene pool and and then these represent the alleles inside that gene pool and so let me start it going for just a second and so inside here we've got the original population if you weren't paying attention I could write some of the stuff out so we had a p-value and a Q value p value is the allele frequency of the dominant in this case let's say the dominant is red and the q value is going to be the recessive and so in the first time we've got 10 and 10 and so our p-value is going to be 0 point 5 and our Q value is going to be 0 point 5 so that being the first generation now we randomly choose or we have sex from jet from that original population to create the second generation or we get this remix into the genes and you can see in the second one that our allele frequency has dropped quite a bit it's now of the dominant we have 0 point 3 and of the recessive we have 0 point 7 on the next one it even drops more now what's making it drop is simply chance so now it's point 15 and point eight five and now it goes to 0 point 2 and 0 point eight and then eventually it goes to zero and 1.0 and so what we've seen is a drift or just due to random size of the population we get the elimination of that red allele or that red color and that's genetic drift if we were to graph that out and we were to put the p value here at point five and point five and i were to graph that let's get it in the right color the p value went from point five down 2.3 down 2.15 and then up the point 2 and then eventually it dropped down to zero so this would be 0 here and then if I do the blue color on the blue color went all the way to 1.0 and kind of mirrored that when like this like that like that and then eventually one like that and so when we graph those allele frequencies over time what we would find is that the values would tend to drift in other words just due to random chance those allele frequencies are going to change like that or move in different directions and so let me give you some actual data based on that let's look here on the internet and so this is a simulator it's a population simulator and so what i can do is i can actually have it do the coin flips inside it and so we're going to start with a population of 50 right now and if I just let it run you'll see that there's going to be random drifting and so we could think of this as the p value and this is the q value you can see that it eventually eliminated over time now let's try to increase the number though so let's say we go from 50 to 500 and we reset the population this time you'll see that there's actually less drift and the reason why we still get a little bit of drift in here is that you have a law of large numbers taking over and so it's less likely to get that change just due to chance but let's really make the number large let's make it like 5,000 and if I click on go you can see it's going to take a lot longer for my computer to actually do this because it takes a while to do all those coin flips but you can see that there's very little drift and so one of the things that's going to keep a Leal or a gene pool the same is going to be the law of large numbers so let me talk about what I'm going to talk about so in this we know that the gene pool is should remain at equilibrium and if we ever change our gene pool we call that evolution and so in the first two podcasts I talked about natural selection and how natural selection can cause changes in the gene pool in this one I'm dealing with more of these ones up here these things that are based more on random chance because random chance has a lot to do with natural selection as well and the big thing I'm talking about is the size and so if we decrease the size then genetic drift starts to take over so for example let's say we have a population right here and then we have a smaller population that breaks off of that maybe they're birds that are blown to an island now chance is going to take over and so these two populations are going to start to vary to real-world examples of genetic drift are the bottleneck effect bottleneck effect is when you have a large population in that large population kind of gets squeezed through a bottleneck in other words the population is going to get smaller even though the population may recover on the other side of that bottleneck there are going to be huge implications as a result of losing all that genetic diversity an example I'll talk about is the almost extinction of the northern elephant seal and that's a human cause the next thing I want to talk about is the founder effect founder effect is similar but essentially it is one founding population making up the the forming population so I'm going to talk about an island in the pacific called pinga lap where a founding population made some huge changes in the phenotypes of the offspring that came from that so this would be genetic drift for a population of 20 as we increase the size you'll see that less of that is going to be the change and so when we talk about an isolated population we could say that this here represents a gene pool and then all of these colors inside it are going to be the alleles and maybe it starts with a 50-50 frequency to start with but once you break off and have smaller isolated populations from that chance can just take over and so you could have the elimination just due to chance not new to adaptation at all and so example of where this might play out let's say this is a population of tortoises that float to an island they're exactly the same as the tortoises on the mainland but due to just random chance you're going to get changes to the point where this actually becomes a new species now there could be adaptation here as well but just chance in itself has huge implications when it comes to populations let me give you a couple of real-world examples of the bottleneck effect and the and the founder effect bottleneck effect remember occurs when the population gets squeezed through a small bottle neck so these are elephant seals elephant seals were hunted almost to extinction the northern elephant seal live along the west coast of the u.s. down through into Mexico and their population had dropped down to a some scientists think maybe drop down to the point of there were just 50 left on one island mexico and so this is in the 1800's and so their population was squeezed through a huge bottleneck if you look at the southern elephant seal the seller southern elephant seal lives in way down here by Antarctica and didn't see that pressure they look very similar they just live in different areas so they're very closely related but some scientists and you can see the citation right here some scientists took a look at the bottleneck effect and what effect it had on the northern elephant seal its population remember had gone from a few hundred thousand down to 50 and then had gone up to I think now it's over 150,000 left so had been squeezed through this tiny bottle neck and so what they did is they actually looked at elephant seals before the bottleneck and after the bottleneck to see how they were affected now you might think to yourself how could we do that how could we look at them before they actually went extinct which fit was in the 1800's well what they did is they grabbed a number of skulls from the Smithsonian so they've got about 11 skulls pre bottleneck and then they were able to extract DNA and so in this case what they did is they looked at the DNA they looked at mitochondrial DNA that's the first thing they did in their study and what they found is that they had lost a huge amount of genetic diversity in other words when they looked at these pre bottleneck northern elephant seal DNA there was a huge amount of diversity but after the bottleneck they're very very similar the second thing they looked at was they wanted to look at symmetry they wanted to see if that actually can affect the skull itself can it affect what they're like can it make them less fit you know typically and so what they did here to kind of put out the data is that they had um here's southern elephant seal here's pre bottleneck northern elephant seal and then here's post bottleneck northern elephant seal and what they were doing was looking at symmetry symmetry of the skull does the right side look like the left side and so they measured this value right here on the lower mandible and they graphed the left mandible distance versus the right mandible distance and you can see that the southern elephant seals there's a really nice linear relationship they then look did the same thing with these old skulls from the Smithsonian and they found that there's a linear relationship as well and you can see the r-values close to one which means it's really close to a linear relationship but then they looked at the symmetry of the post bottlenecked northern elephant seal and they find that that data is all over the place in other words their skull is very asymmetrical what does that mean well when you decrease the DNA you're actually having it to manifesting itself on the morphology or the outward appearance of the skull and so that could make them less fit to changes in the environment or more susceptible to another extinction or another bottle necking effect another example related to humans would be the founder effect so the founder effect can happen in any organisms but the founder effect is when you have a small population that finds a new area or is the founding population of that new area and so I didn't know about this but maybe you did in the in the South Pacific we have what's called the Federated States of Micronesia and in there is a tiny little island called pingel app and it was doing well back in the day but in the 1700s they had a massive typhoon and that typhoon swept through pingel app and as it did that it killed almost everybody on the island so their population dropped down to around 20 so those 20 people were the new founding population of pinga lap and all the people who live there today are descendants of that first 20 people now what's interesting is that the leader of pinga lap I'm not going to try to pronounce his name but the leader of pinga lap one of these 20 survivors actually had an odd form of colorblindness where he had complete color blindness in other words when he looked at a macaw like this he sees it in grayscale couldn't see any color at all this is really rare something like one and every thirty three thousand people in the United States have this but since this was the founding population very quickly five percent of the people and now I think the numbers up to around ten percent of the people on pinga lap have complete color blindness and that's due to just the random chance in that founding population now the number is going higher than it was originally and so that suggests that there's inbreeding as well and if this is the leader of that population that could have been quite a bit of inbreeding as well and so that's an example of a fan ending effect it's just the chance of who happens to survive or who happens to land on that island or in that one area that can create huge repercussions for the for the rest of the the time of that population and so that's a genetic drift again it's just randomness but it gets bigger and bigger and bigger the smaller the population is I hope that's helpful