one of the assumptions of the hardy-weinberg model is that we have a large population and the reason is that in a large population chance events are not going to have much of an impact on allele frequencies if the population is small say 10 to 20 individuals or even 50 to 100 or even a few hundred individuals it's much more likely that just by chance the allele frequencies uh are going to change over time just based on which alleles happen to appear in the few individuals that we have in the population some of you may have taken the biodiversity lab and have participated in a population genetic simulation in which you you tossed a coin to simulate the outcome of of meiosis and then simulated random mating uh without selection and then did it again with selection and uh in that simulation um the population is small um you know maybe a couple dozen students at most and so sometimes just by chance you know when you toss the coin more more coins come up heads sometimes more coins come up tails and you're not going to always get 50 50. you know sometimes those coins are going to be skewed quite far one way or the other and so in a given generation because there's so few individuals in the population the frequency of one allele or the other can fluctuate up or down quite significantly but if you were to do that simulation with say a thousand students you would be tossing hundreds of coins each generation and it would be unlikely that the the ratio of those tosses would deviate significantly from you know the expected 50 50 ratio given the large sample of coin tosses so in a large population allele frequencies are not going to fluctuate based on what the alleles do in meiosis or what sperm fertilizes what eggs we've got a big enough sample in a large population that chance will is not going to impact the frequency over time now sometimes real populations are small and in that case chance has an effect and when chance has it has an effect like that that process is genetic drift and and we can refer to this also as what's sometimes called a random walk and if the allele is neutral in its effect if there's no selection acting on it if it doesn't affect fertility it's going to fluctuate randomly and so given enough time the allele will eventually be either eliminated or fixed it's going to go one way or the other and you may have seen this in your simulation in the biodiversity lab you may not have gone to fixation or or elimination but you may have seen it going um in that direction it's kind of an analogy of a drunk walking down a railroad platform he's uh so far gone that he he can't correct his uh his the direction he's walking his next step is basically random other than that he's more or less going forward but he's going side to side more or less randomly and you know eventually he's going to fall off onto the tracks either to the left or to the right it's either going to be fixed or the allele is going to be uh going to be lost now you know it may depend on how wide the the uh platform is or how far he has to walk if the platform is narrow he's not going to get very far if the platform is wide he might have to walk aways but eventually given enough time that allele if the population is small is going to go one way or the other and so the if the population is really small that would be like a very narrow platform that he's walking down and it's not going to take long before he goes off one way or the other if the population is a little bit larger he might have some leeway he can sway back and forth one way or the other but eventually he made 100 yards down the platform he he's gonna stumble off on one direction or the other so it'll occur quickly in a small population narrow platform or more slowly in a large population also the probability of elimination or fixation is dependent on the the current allele frequency you know we might kind of as a starting point think of a allele frequency of 50 we have 50 of one allele and 50 of another allele but that's probably not usually the case you know we might have 95 of one allele and five percent of another allele and if that's the case then obviously the allele that is at 95 is more likely to be fixed and the allele that is that five percent is more likely to be eliminated although if the population again is small the drunk is starting out on the left side of the platform he's more likely to go off the left if it's narrow but if it's a small population if it's a very narrow platform just by chance he may stumble a few times to the right the coins may come up tails instead of heads and he may end up very quickly on the right side and genetic drift will cause the population to go the other direction but the probability is if you start out with that frequency that's the direction you're going to go deem i think we have not talked about deems but a deem is a small sub population we've talked about populations all you know a lot but a deem is a subdivision of a population a smaller group of organisms that are very likely to interbreed with each other and more likely to breed with each other than with the larger population outside so this beam is is a is a smaller group that um has has some of the properties of a population and there's some there's there's more likelihood of breeding within the the dem than outside of the deem and consequently that within the deem within that small sub population there are uh there's a possibility of allele frequencies being affected differently within the team than in the population at large