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in our last video we did three examples of hardy-weinberg equilibrium and took you through the whole process but it really is worth practicing again if you feel confident you know how to do it exactly you can skip ahead and just go to the PowerPoint part of this video but I think it's worthwhile especially if you're having a little bit of trouble so I've got two examples we're gonna work through the first one we go into a population and we sample a thousand individuals we find 360 individuals are big a big a 480 are heterozygotes and 160 are homozygous recessive or little a little a so your goal is to take these genotype counts turn them into oliel frequencies those are your P and your Q values take the P and the Q values and plug them into the hardy-weinberg equation and then determine whether or not this population is at hardy-weinberg equilibrium pause the video now and then we'll go through the results together so this is what you should have ended up with I do two times 360 because there are two copies of the big a allele and these 360 individuals I add in 480 because the heterozygotes carry one copy I get the total number of big a alleles I do the same and get the total number of little a alleles I then divide by the total which in this case is 2,000 alleles though that number comes from 1200 plus 800 and I get 0.6 in point four okay that's my p value my Q value I plug them into this hardy-weinberg equation do a little bit of math and I end up with predicted genotype frequencies for the next generation so remember this first number is a prediction for P squared which is a prediction for the homozygous dominant two PQ a prediction for the heterozygotes and then Q squared a prediction for the homozygous recessive okay now at this point I can check to see whether my predictions match my original observations and in this case they do 36 percent or 360 out of a thousand 48 percent for an 80 out of and so on so for example number one we are in hardy-weinberg equilibrium so now let's take a look at example number two again the same thing go through the same process get it all into the hardy-weinberg equation and then determine whether or not this population is in hardy-weinberg equilibrium pause the video now and come back and check your work so this is the end result same exact process right so I won't take you through all the details because we just did it but look at the results we predict a little over 30% of the population should be heterozygous dominant but it's actually 48 percent we're predicting a little under 50 percent it should be heterozygous but we only have 16 percent and then of course this is number is out of balance also so this population is not in hardy-weinberg equilibrium okay so for the rest of the discussion let's talk about the things that cause populations to not be in hardy-weinberg equilibrium so that equation that prediction becomes a necessity a null hypothesis which is if there are no evolutionary forces acting on a population everything for that allele should balance now it's important to remember when we're doing hardy-weinberg equilibrium we're only looking at one gene the two alleles for that one gene and other genes could be changing and evolving but other other alleles might be in equilibrium and stay exactly the same generation after generation so this null hypothesis is a prediction for which the next generation will be and it also tells us if there's any evolutionary pressure that's acting on that gene so there are three things that sorry five things that can impact hardy-weinberg equilibrium and throw it out of balance or violate our no null hypothesis okay so here they are now three of these you are already very familiar with natural selection genetic drift which is small population size that's one genetic drift is in a in effect so in fact let's I'm gonna so hopefully you recognize why but if not we'll get to that in a later chapter but we're gonna replace small population with genetic drift and then of course new mutations if those things are happening to an allele then we will see imbalances they may be slight or they might be large there are two other things that are added now some people will put these two other things under the umbrella of natural selection but for our purposes we're gonna separate them so non-random mating that means if you are choosing a mate based on the the gene that we are investigating and we'll look at examples of that in later chapter and then migration if we have individuals migrating from a different population with different allele frequencies that can throw our balance off now it's important to recognize that there are many many different balance points for the hardy-weinberg equilibrium this is a three-dimensional representation of all of the possible outcomes of the hardy-weinberg equilibrium and it looks a little bit like a halfpipe right the skating halfpipe so if you throw a marble into a skating halfpipe and if it was perfectly balanced on the bottom there are multiple places anywhere along the bottom of that trough you can have a balance point and the same thing with hardy-weinberg you can balance with 50:50 you can balance with 2080 you can balance with 3070 anything as long as all of these forces are not acting on the population you can reach balance at many many different points okay so that's important to remember so let's look at some of these things mutation on hardy-weinberg we're just gonna treat very very quickly because mutation is random and unpredictable and so we can have all sorts of different new alleles but that complicates things if we have a new mutation and suddenly we have three alleles instead of two alleles the hardy-weinberg equation becomes more complex more complex than we want to deal with for this class okay but I do want to generally talk about it because new mutations are happening all the time many of those mutations are negative right or deleterious they have a negative impact on the individual that carries them but then we have forces that try to remove those negative mutations from the population and by remove I mean individuals carrying mutations don't do as well they don't have as many offspring and so that mutation is underrepresented in the next generation and if that continues eventually we will remove those negative mutations completely from the population so deleterious alleles or negative mutations reach a balance that depends on two factors number one is the rate of mutation or how many mutations occur each generation and remember that can vary and some organisms mutation rates are high and there's lots of variability introduced each generation and in some populations mutation rate is low but there's always going to be some level it's bringing in new mutations and some fairly significant maybe small but significant amount of those new mutations are going to be detrimental and so we have mutation rate bringing in some amount of bad mutations and then natural selection working to remove those negative selections and so the balance point depends on those two forces mutation bringing in negative things and natural selection working to remove those negative things now there are neutral things and occasionally a rare positive or beneficial thing being brought in by mutation too but for now we're only gonna look at that at these deleterious alleles in that general sense so migration is another thing that can greatly impact populations so let's redefine population and we talked a little bit about this in our very first lecture but let's just clarify it a population is any subgroup of a species so an entire species can be a population but we can also divide a species into smaller populations and there are none numerous names that we use for this in the human population human species we use sometimes race or ethnicity to define a sub population in other groups we might call them a breed right so for dogs we have different breeds that represent different subpopulations of the the dog species plants we might call them cultivars in fire or bacteria we may call them strains but those represent different subpopulations of a much larger population so it gives us a little bit more detailed definition of population populations can be entire species but we can also subdivide them and those subdivisions are based on some sort of genetic separation so here we have two subpopulations in a species one on the mainland with a different allele frequency than one on the island and if we might have migration one way maybe there's a current or common weather pattern winds that make migration only occur in one way if that's occurring that is going to throw off the hardy-weinberg equilibrium in this population on the island now eventually if enough of that occurs it will reach the same equilibrium that we see on the mainland but in the time being until it reaches that point we are going to have a little of an imbalance in the hardy-weinberg equilibrium and so if we put in our numbers and saw that imbalance we would say oh there's something going on here and so when the hardy-weinberg equilibrium when our predictions don't match our original observations we know that there's an evolutionary force acting on a population now it's tricky sometimes to figure out what that force is it might be natural selection it might be genetic drift it might be mutation it could be migration like we have here in this example or it could be a non-random mating okay so let's look at non-random mating so a non-random mating there are basically two general types and lots of degrees in the middle so we have assorted of mating is when like individuals mate with other like individuals we have dis assorted of mating when mates prefer to choose an individual who is different from them so assorted of mating is also known as inbreeding when individuals mate with individuals that are genetically similar to themselves and inbreeding has some negative consequences the most extreme form of inbreeding is self-fertilization alright if you breed with yourself now not all species can do that most animals cannot there are a few exceptions to that rule but many plants can self fertilize and so if we have self fertilization you know you're meeting with yourself that's the ultimate genetic similarity if there is too much inbreeding we get something called inbreeding depression now a lot of people think oh if you mate with other individuals are gonna be more mutations and that's bad it's it's not that there are more mutations but here's the reason why inbreeding can cause problems every individual on average carries two or three deleterious alleles and in fact there's a really good chance that you carry a fatal allele now hopefully that doesn't concern you too much because almost everyone does however you are carrying a fatal allele that is recessive and you means you got one bad copy from one parent and a good copy from another one and because it's a recessive fatal allele it's not a big deal because you have 20,000 some odd genes in your genome there's a very very low chance that your mate carries that same recessive allele unless that mate is closely related to you and then the odds that those rare recessive alleles that are fatal are the same in your you and your mate those odds jump up quite a bit and especially if it happens generation after generation so inbreeding depression the reason that it's a bad thing is because of the pairing of those rare recessive lethal or at least extremely detrimental alleles and that's called inbreeding depression now in many species there are instances of dis assorted of mating one prime example of that is MHC in mice so we've got an experiment here with the birds where if we put a female bird into a experiment and give her the choice of a male bird that looks different than her and a male bird that looks much the same as her she will more frequently choose the male that looks different that's a that's just a mutation that popped up in that population it was a beneficial thing because it reduces the chances of inbreeding depression if females on average choose mates that are different than themselves then they're gonna pick mates that are more their offspring are gonna be more genetically diverse and make seeing mice is another one so MHC stands for major histocompatibility complex in mammals that's an important determinant of your immune system and the more diverse those MHC genes are the more diverse and better your immune system will be and so if we do this same experiment where we put a female mouse into a chamber and let her choose a male that has the same MHC complexes or different MHC complexes she will more often choose the mate with different MHC complexes now how do they know how am i stating that well almost certainly it's a chemical cue and I think there's even been some research finding out exactly how they do this but it's one of the ways that might mice assess mates there's probably something similar going on in many mammal species maybe even in humans a little bit subconsciously how do we assess a mate do we do it purely on visual things do we do it on other communications like you know verbal communications or physical cues is there some sort of a pheromonal thing a chemical cue that humans are using and there is some evidence for that but certainly in other animals that's much much more pronounced this is the reason dogs go around sniffing other dogs butts right to put it kind of I guess it's a little bit crude but not too crudely all right they go around they sniff their butts they're assessing chemical cues is that individual fertile is that individual sick perhaps even other ones is this individual the same as me is different than me and they're doing this all because these random mutations for these behaviors have popped up if they're good they're kept around if they're an advantage to the population