Allelic Frequency Changes

Hi in this video, we're gonna be talking about a little lick frequency changes. So hardy Weinberg, hardy Weinberg exists in this ideal world of these five Samir assumptions. Um But reality isn't like that, right? And these Samir assumptions are way too restrictive to exist in real life. So we're going to go through each five of these and talk about what happens in real life population. So the first one is selection, right? No selection for hardy Weinberg. But in real life there is selection. And that selection is called natural selection. And natural selection is when organisms have genes or leal's that allow them to survive better and allow them to reproduce. And the reason that natural selection exists is simply because there is a struggle for survival. So there are many organisms that are born on this earth. There's way there's so many organisms born on this earth, not all of them can survive. Because not all of them can survive. There have to be a few that are more apt to survive and reproduce than others. And those few usually have some kind of genes that allow them to survive and reproduce better. So, because not everyone survives the ones that do have particular phenotype that allow them to and they can pass that on to their offspring. And that's how pretty much selection works. Now, there are many different types of selection. One is called directional selection and that is when an illegal moves in one direction towards what's called fixation or loss. Loss makes sense, right? That Aaliyah will be completely lost from the population, but fixation. What does that mean? The exact opposite of law? So fixation fix the wheels are leo's found in every organism. So when an allele is fixed, that means every organism in that population has only that one allele, there's no other legal option for them. And so directional selection moves what could be two or three or four alleles towards one direction where they are fixed in a population or they're lost from the population. Then there's positive and purifying which are two separate selections. So positive selection brings. So what happens in both of these is that there's some kind of mutation, right? So when there's a mutation that can either be beneficial or it can be harmful or it potentially could be neutral. But in selection it either has to be beneficial or harmful. So when a mutation is beneficial, positive selection says, okay, that's great, we need this, it's beneficial. It helps the organism survive and reproduce. So let's make it a higher frequency purifying. Selection says that mutations harmful. It makes the organism less likely to reproduce. So we're gonna get rid of it. We're gonna remove it from the population. And then finally there is balancing selection and this is when there are two or more levels, but essentially they're both useful, they're both beneficial to the population. And so they are maintained at some kind of equilibrium where they both exist and um selection sort of weeds out many of other wheels for these, You know, to balance the majority of the organisms have these alleles. So an example of this would be directional selection where the red represents before. So this is before we're here and the blue represents after. So before directional selection, there's a really high amount of this one phenotype less of this intermediate. And a very low amount of the phenotype. After selection. This one the phenotype one has been lost and this one has been gained moving towards fixed. Right? And um after this means that there's a low amount of phenotype, one intermediate of intermediate. But a very high amount of phenotype too. And that's due to this directional selection where this alil becomes lost and this one is gained at a higher frequency. Then they're stabilizing selection. This is also kind of like balancing selection where before there was sort of this broad um lick frequency distribution over three different types. Whereas after the selection that the alleles for this intermediate phenotype here in the middle has been selected for. And that intermediate phenotype is usually a mixture of multiple alleles. And so those multiple alleles have been selected for. Now let me disappear. So we can talk about this. Another measurement of natural selection includes fitness and what fitness is. It's a measure of how well an individual's genetic makeup contributed to uh next generations. And so that says, how well are those genes passed on? There's two kinds of fitness is there's absolute fitness. And this just says the number of offspring and individual has, right. The more offspring, the more genes you passed on. The second type is relative fitness. And that's the fitness of the number of offsprings and individual have compared to another individual. So, if we say that we have a brown fish and a blue fish, and the brown fish produced two offspring, and the bluefish produced 10, well, the relative fitness is 2-10. Right? And so that compares how fit the bluefish was to the brown fish. That says brown, sorry for my handwriting. Um And so fitness is another way to measure selection. Now, selection exists, right? Natural selection. This exists all the time. We can actually see it in our lifetimes. And it is a reason why because this exists is also why hardy Weinberg equilibrium is not a good representation of current populations living at the time. And instead is just an estimation. So that selection now, let's turn the page

Okay. So now let's talk about the new alleles and the mutation which are the A. And the M. Of Samir. So new alleles are created all the time and those change the frequencies. And so one of the main ways that new alleles are created is through mutation which is happening consistently and so mute. We can create new alleles in a population but it also can convert one allele into another. And so to measure that the creation of new alleles. We actually look at the mutation rate which can just be calculated without knowing anything about evolution right? It's just calculating the mutation rate of that organism. And this is the rate at which mutations occur in the population. And there are various formulas that you can use to calculate the mutation rate and how frequently that will introduce new alleles. And also this formula here can calculate how the mutation rate affects um mutation rate on A li L. P. Causes the change in the frequency of Q. Because that mutation is changing the llp into the L. Q. Now some of your books are very heavy on these types of calculations but most of you will get lucky and won't have to calculate these yourself. Just know that alleles are created all the time. And this is often through mutation the third way or the third letter. And Samir is m its migration. It's also called gene flow. These are the exact same thing. So whichever your book use make sure you use that one and that's the movement of individuals between different populations. You can also use sub populations right? Because it's it's different populations. And so when one sub population sort of, there's an individual that migrates into the next one that can create this um concept called genetic admixture. And that is a mix of genes in individuals that arose from multiple populations. Right? So if um for instance if we have this uh black circle population and this red circle population, when the black circle population and individual moves over here and mates, then that's going to create this genetic admixture in the offspring. Because that genetic information is coming from more than one population. And this allows ads genetic variation in the population that does not meet the exceptions necessary for hearty wine equilibrium. The hardy Weinberg formula. So the introduction of new alleles either through mutation or through migration or gene flow are super important things that are happening all the time. You know, mutations happening all the time. And individuals in different populations are constantly migrating um throughout different sub populations. And that's creating a lot of genetic variation in um these populations. So with that, let's not turn the page

Okay. So now let's talk about genetic draft. And this is for the fourth thing of Samir, which was infinite population because the population isn't infinitely large. Right? Population can't be infinite. There isn't this like infinity number of people in a population. And so because there is a restriction on population number, it can't be infinite. So because there is this finite population That then only so many offspring will be produced. And those offspring only represent a subsection of the total of eels and a population. So what this means is that even though population is infinite or because the population is finite, there's a limited number of gambling, say 3000 jamming per person, which is actually much lower than what it is. Well, not every gamut that's created produces an individual and because not every gamut produces an individual, then each generation only contains a sample of a little. So a small subsection of alleles from the the previous generation are passed on to the next. And those are the ones that stay in the gene pool. But because there is a loss of alleles, essentially, not every level has passed on. There's the same called genetic drift. So, how this happens is we have so many alleles in our if it's for us, right, just take yourself as the example. You have so many alleles that represent your genetic material. Now, when you create gamut, so those primordial germ cells, right? Either for sperm or eggs. There's a random selection of alleles. Remember there's random assortment. So you only get half half of your total alleles in every single egg or sperm. Now, there are millions of eggs and sperm produced, but we don't have millions of Children. So only a very small subsection of all the meals that we have will actually be passed on to the next generation now. And that's completely by chance. Just randomly chosen by just whatever. It's just a random selection of alleles that are made on to the next generation. However, the more offspring that we have, the more alleles that are passed on, and this is again completely all by chance, right? There's just chance of these alleles get sorted into that gamut. It's just chance that that gamut goes on to actually produce an individual offspring. So, if the comedic sample is small, so the number of gametes that are actually produced, right? Which is true for humans. In our case, then a small number of offspring is produced. And that means that the greater the chance that gametes will differ from the entire gene pool. So there's this whole parental gene pool that contains all the genes from both of our parents. But we only get half of those. And though those, there's a great chance that those half are going to differ greatly from the frequencies found in the parents. And so this was referred to sampling error, right? And I guess the best way and this is sort of a deviation from the expected ratio due to limited sample side. So let's say that I have 10. Um or I have five pennies and five nickels. Right? And I'm going to have choose two of them. There is um, only a small chance that I'll choose one penny and one nickel representing the frequency that was found in the beginning, right? There's a high chance that I would choose to pennies or choose two nickels. And so that would be an example of sampling error. And the same thing happens with the alleles. So we have so many levels to choose from with parents. We only get half of those and only some of those produced into an individual. And so, um, because we're only making small numbers of individuals, then there's a very high occurrence of what's called genetic drift. And genetic drift is this change in a lilac frequency. So like I said, the frequency from half and half pennies, half Nichols changed to two pennies or two nickels when I chose that sample. And so genetic drift is the same just with a little, So genetic drift is the change in frequency due to these just random choices, These random disappearance of genes in the small population. So genetic drift happens at a much higher rate and small populations. And when a little light frequencies are equal. So, um, here's a pictorial example of it and that is say this is exactly the same with our pennies and nickels example, except for we have red circles and blue circles. So we start out with the same number of red circles and blue circles in this first generation, Then just due to random chance to sort of random gametes being formed and random gametes being produced, what happens is we actually end up in the next generation with more blue than we did with red. And so this is a change in allele frequency. So the allele frequency before was 50, 50. And now it's not. And because this keeps happening through the next generations, this is a generation. This is the Children, the grandchildren and great grandchildren. Eventually genetic drift. This sort of random changing the frequency keeps happening where we keep getting these blue heels more often and it's just happening by chance. Nothing selecting for this, right? There's no natural selection here. It's just by chance were randomly choosing the blue more often because we randomly choose the blue more often, we can end up with all blue in this later generation. So that's genetic drift. And so when populations, so how does this refer back to this infinitely large and hardy Weinberg? Well, when populations are infinitely large, genetic drift doesn't occur. But when there's a finite population and so we only have these to choose from the genetic drift is going to happen because it's just a random occurrence were randomly, sometimes going to choose blue more than red and sometimes we would choose red more than blue. And it works with different alleles and different genes. And so in finite populations, especially small populations that produce a small number of offspring. Um this happens at a very high rate. So genetic drift can lead to fixation or loss of an allele. And we talked about fixation before, but I want to just remind you that this is an example of fixation. And fixation occurs when all individuals in a population are homos, I guess for one allele. So all of the individuals in this general, I have two copies of this blue allele. Therefore it's fixed loss occurs when no individual in the population carries the legal, there's no more readily tells the red illegal has been lost. So genetic drift can lead to this fixation or loss of alleles in individuals Now. Um and that of course leads to genetic variation. So genetic drift can we talked about it in terms of this random occurrence of these genome or these organics that are produced. But genetic drift can also, because by these two major occurrences. The first is called the founder effect. And this is when a new population of a much smaller size is formed from a founder. So we started out with a population of 5000 birds and And a sub population of them say like one or two of them moved to a different location, started a new population that that there's two birds that were chosen out of the 5000 to begin with don't carry every allele that this 5000 did. Right, it's impossible. They can't carry all those alleles. And so they lose that variation When they create this new population that then can go on to create 5000 more birds. This is a definitely genetically restricted population compared to the population that the founders came from. So because these founder birds can't contain all those alleles, that reduces the variation found in their offspring as well. The second form is called the bottleneck effect. And this is just a contraction and population size. It can happen in one generation. It can happen in multiple generations. Example, people like to use this for instance, polar bears, which has lost a lot of their environment and um are essentially starving to death. I mean, as horrible as it is, it's true. And um so they're experiencing this bottleneck effect where there are losing a lot of individuals over the course of these many generations. And so even if everything was perfect for polar bears, again, there's been a huge loss in alleles in the population. So there's not as much variation. And when there's not as much variation that definitely restricts the genetic diversity that we can see in those organisms. So, an example of a bottleneck effect is here. So you start out here, and we can just say, you know, these circles all represent a different type of allele and we can even represent them as polar bears or whatever organism you want to represent. And so something happens in the environment or maybe there's a disease or they've lost their habitat or they're not getting enough food and they're this big die off of the population and it can occur over multiple generations as well. But essentially what happens is you started out with this big diversity of skills and the bottleneck has restricted that. And so you have much less numbers, but you also have much less diversity. You see there's much more red and blue, then there is pink and eventually that pink is lost. And we start getting this red that's chosen more and more often. And so this was very diverse. And then after the bottleneck is not diverse at all. And so that leads to a loss of genetic variation and which is very detrimental to the organism itself and its ability to survive and adapt to other future things that could occur. So that's genetic drift and that occurs in non infinite populations, which are the assumptions of hardy Weinberg. So with that, let's now move on.

Okay, so now we're gonna talk about non random mating which is the last part of semi are the M. And so in hardy Weinberg, there has to be random mating but in real life that doesn't actually happen. And non random mating due to phenotype sis which are caused by different alleles. Oh my goodness, I really need to be better writing our occurs in every organism on earth including humans. And so it's called a sort of mating. When individuals choose mates based on phenotype and there's two types positive and negative, it's positive mating um when it's chose chosen based on similar phenotype, so how similar the two mates are typically and it's negative when it's based on dissimilar phenotype. So when they differ and so this actually happens in humans. And I love this study. And so it turns out, so what they did, they wanted to see if female and males has any kind of um sort of a sort of a sort of mating based on smell. So what they did is they got males to wear white t shirts and they lived in these shirts essentially for a few days they exercise, they got them real sweaty, real smelly, it was nasty. And then they put them in plastic bags and they gave them to women to rate based on attractiveness. And so it turns out that women prefer the the odor of males who have a certain genotype and that genotype is very different from them. So the deal that they were looking at were these alleles called NHC you may or may not know what they are. It doesn't really matter what they are. Just know that they're involved in the immune system. And so females always chose men with different MHC molecules and they could tell this just simply by the odor that those MHC molecules were different in the men. And so there's obviously something in the odor of probably both females and males that allow humans to differentiate different potential mates. And so, um, this is an example of non random mating. And there's examples of this all throughout the animal kingdom, different mating dances, uh, feathers or colors or whatever locations, geographical locations. All of this is an example of non random mating. So, um, there is another, so that was kind of um based on phenotype, but there is actually another type of non random mating, which is isolation by distance. And so two populations that live in different areas that are either separated by continents or countries or even something as simple as a mountain or a stream um aren't going to mate with each other because they can't get there. They're not like humans with planes that can fly all over the world. Generally, organisms are restricted to a very small geographical location. And when there's two populations are more populations that are separated from each other for an extended period of time due to selection or genetic drift or any of these things that we've talked about previously genetic variations begin to develop between these new populations. And when those variations get to a certain point, we call the speciation, which is the creation of a new species. And speciation occurs through reproductive isolation. So isolating to populations that now can't reproduce with each other. There's two types of reproductive isolation. There can be pre psychotic. So this is before the formation of the zygote. So something that's prohibiting them from mating. Um and that reduces breeding and so try to think of some things that would be pre zygomatic, prevent the organisms from dating right? It could be isolation by distance, which is what we're talking about. It could also be that the organisms don't have um the genitalia to be able to make or that they would never think of each other as um as mating populations to begin with, for instance, having birds and bears are never going to look at each other and say that's probably a good mate. And so those are all pre psychotic mechanisms. Then there's post psychotic mechanisms and these are things that after the zygote is formed prevent further reproduction. So usually what happens is there's some kind of offspring created from these mates, but they're either in viable meaning that they die or they are born but they're infertile because they're sterile and they can't reproduce. If you can't make more offspring, then is considered reproductive isolation. So an example of this as a mule, which is a meeting between a horse and a donkey mules are example of post psychotic isolation because they're infertile and they cannot reproduce and so you can't get more mules by mating mules and therefore it's an example of reproductive isolation. So here's an example. So you start out with this um one feces a barrier is formed. Either a new stream or some flooding or maybe they just move or something happens is the barrier formed. And in this isolation they actually end up becoming two separate species so that even if they begin to overlap again, say the barriers removed and they begin to overlap now that they're two different species, they won't reproduce together anymore. So that's speciation and isolation by distance. Another form of random mating or non random mating is inbreeding, which I know is kind of a taboo subject among humans. But essentially it's the mating between relatives and inbred individuals are much more likely to be homos, I guess for harmful recessive alleles, right. A lot of genetic diseases are recessive genetic diseases and inbred individuals are much more likely to be homos, I guess for those and are much more likely to have recessive genetic diseases. And so it's called inbreeding depression when inbreeding leads to reduction and bigger or reproductive success, which would happen in the case of a recessive disease, right? If somebody has a recessive disease, they're much less likely to reproduce. Um and be healthy in general enough to reproduce. Now we usually think of this as super taboo in humans but actually inbreeding in plants especially through self fertilization can actually be the positive process. Um But generally this is inbreeding is non random mating right? Because your mating with your family now you can measure inbreeding through inbreeding coefficient and it's it's f And that's the probability that two alleles in an individual trace back to the same ancestor. And um of course inbreeding is much more common amongst all populations because there's not that diversity of choice of mates right? It's very limited and so it's much more common there. So the inbreeding coefficients for something like a father daughter is 25%. So there's a 25% chance that if you choose to alleles from this from this offspring is going to go back to the same ancestor. And that's very high especially when you look at recessive genetic diseases that's a really high inbreeding coefficient. And you can look through the rest of these and see that you know as you get further away the second cousins for instance it's 1.56%. I don't know why these don't have percentages on them. But there we go. Um And so all of these uh you can look at each one of them but essentially inbreeding an example is an example that happens especially in the animal kingdom more often than you would think. Um That is non random mating. So that is Samir. S. A. M. I. S. A. M. I. R and and that is how all these assumptions that hardy Weinberg make, makes in its formulas aren't necessarily that relatable to real life situations. So with that let's not move on.

Which of the following terms describes a change in allelic frequency due to random disappearance of genes in a small population?

• A. Natural Selection
• B. Allele creation through mutation
• C. Gene Flow
• D. Genetic Drift
• E. Non-random mating

A group of finches live on a small, isolate island. One day, a few finches travel to a distant island and start a new population of finches. This type of change in a population is called what?

• A. Speciation
• B. Founder effect
• C. Bottleneck effect
• D. Genetic drift

Which of the following is an example of natural selection?

• A. A neutral mutation is carried from generation to generation
• B. A rabbit migrated to a new location and brought new alleles to the endogenous rabbit population
• C. A mutation causes a finch to develop a stronger beak, which makes it more likely to grow, survive, and reproduce.
• D. One allele becomes fixed in a population due to random genetic drift over time.