Genetic Variation

hi. In this video, we're going to talk about genetic variation and populations and different types of natural selection Now, even though the goal is usually to increase genetic variation because that tends to help populations, sometimes there are circumstances that actually decrease the genetic variation in a population and in breeding is a great example of this. Inbreeding is mating between relatives, and it tends to increase homocide gossett e in the population. And this can also lead to something called inbreeding depression, which is a decline in the average fitness. As homocide, gossipy increases and headers, I gossipy decreases No home, ozai gossipy. Increasing would mean that there is less genetic variation, right, because that's going to mean that one Lille in particular is going to be pretty dominant in the population, dominant not necessarily in terms of whether it's dominant or recessive, but dominant in the sense of it's going to be prevalent. It's going to be wide spread in our population. So here we have a nice model of a pedigree for Shetland ponies, and you will see that there is the capital, a dominant Lille, this lower case, a recess of Lille and over generations when you have inbreeding that tends to result in more Hamas egas individuals were as out breeding tends to result in more hetero zegas individuals. Now it is advantageous generally to have more hetero six individuals and to increase the genetic variation in a population home is I gossipy tends to decrease fitness in the long run. Now there are exceptions to this. I'm just talking about general trends here and in breeding is why it's so difficult to bring back species that are on the brink of extinction because their populations air so small. It's really difficult to avoid inbreeding so generally when trying to revive those species those endangered species. Inbreeding happens, and the overall fitness of those organisms in that population tends to decrease. Its just one of the many challenges that conservationists have to face Now sexual selection is a type of natural selection. That's where some individuals out compete, others because they're better at securing mates. And often this can pertain to you sexual dime, or if is, um, which is a fiend tip IQ difference between the males and females of a species, and you're probably very familiar with this in terms of bird mating, which is a great example of inter sexual selection, which is selection between the sexes. Usually when a female chooses a male and many birds display a great degree of sexual dime or if ism or a great degree of variation between the appearance of the males and females. And usually this comes in the form of males having very decorative perhaps so kind of lavish or grandiose appearance that they use to attract mates. So here you see an example of the peacock, with its very large, impressive tail feathers and the well, no offenses rather unimpressive Pea hen, the female of the species peacocks and pea hens have a use, a form of inter sexual selection. Females display their impressive feathers, and the females decide whether or not they're impressed. Now you can also have in trust sexual selection and in trust. Sexual selection is selection within a sex, and this is usually in the form of males competing with one another for mating rates. Like you see, these two elephants seals right here that are squaring off for a fight, they're competing for mating rates. Now, the interesting thing about inter sexual selection not backtrack, too much is that inter sexual selection can actually result in some kind of funny appearances for organisms. So you might think peacocks feathers are kind of lavish and grandiose and guess what they are. They're a little ridiculous, right? Why does a bird needs such crazy tail feathers? Well, the reason is that the females tend to pick males who have the most impressive display of plumage, meaning that over generations the males of the species that are going to be producing the most offspring because they're going to be securing the most mates are those with the most impressive tail feathers, meaning that over generations the males of the species are going thio. Because of this inter sexual selection, they're going Thio Thea Leal's In the Population are Going Thio three Wheels form or impressive tail feathers air going to increase. So over generations you're going to have a larger and larger percentage of males who have crazier and crazier tail feathers, and this can actually go awry. This can actually go awry for organisms. There is an example of a particular type of extinct dear who scientists think mating waas, a type of inter sexual selection For this organism and these deer. Their antlers grew so big they grew bigger and bigger and bigger because the females they picked the males with big antlers. So over generations, the males evolved to have bigger and bigger antlers to try to compete for those female mates. And it got to the point where their antlers got so ridiculously big that it was actually very hard for these organisms to even lift their necks. It basically the inter sexual selection pressures made drove this species to extinction because the males of the species eventually ended up with antlers that were so big they couldn't even function properly. They couldn't live normal lives. So it just goes to show that sexual selection isn't always driving organisms to be more fit. Sometimes it actually hurts the fitness of those organisms, whereas intra sexual selection tends to favor males. You know, again, we're just speaking generally Now it tends to favor males who are more fit, who are bigger, stronger mawr Oh, are bad Skerry, right? The guys who are gonna win the fight. Now let's flip the page and talk about some other types of natural selection

in directional selection, the average phenotype of a population shifts in one direction, usually favoring an extreme phenotype. So imagine that most birds in a particular population were sort of a mid size between this small bird and this big bird you see over here, there were some sort of mid size. Now, directional selection would push and cause an extreme phenotype to be favored so that the average size of the birds in this population either get much closer to these small birds or the's big birds so it can be pushed in either direction, depending on the pressure from the environment. So you also will sometimes see stabilizing selection. And this is when extreme FINA types like the really small burden Really big bird. Those extreme FINA types are selected against. So actually, the average phenotype doesn't change. The average phenotype stays the same. However, genetic variation is reduced. So we go from this outer red curve here to this inner blue one. The average this mid middle of the road phenotype stays the same, but the percentage of the population that has more extreme FINA types reduces. You also will sometimes see uh, stabilizing selection due to a purifying or negative selection, and this is the removal of deleterious Leal's or a Leal's that have harmful effects on the organism. So purifying selection doesn't always lead to stabilizing selection, but it can. And that's usually when those deleterious a Leal's result in some extreme FINA type. Now, lastly, you can actually have disruptive selection. And that's when the extreme FINA types are favored over the intermediate ones. But both extremes Air favored. So, unlike directional selection, where we push either in one direction or the other here in disruptive selection, we actually go for both extremes. So we started off with this red curve, and we wind up with this blue curve in terms of the distribution of phenotype. So here the mid sized birds are being selected against, and small and large birds are being favored. Now let's turn the page, talk about some more types of natural selection

balancing selection is when No. One Lille is favored over the other. And this can sometimes be due to what's called the hetero zegas advantage, where hetero zegas individuals actually have an advantage over Hamas. I guess ones now. A great example of this is what the condition called sickle sell anemia. Oops and sickle cell anemia is caused by having two copies of a recess Ivo Lille, for a particular gene. Now you would think that because this disease is very, very harmful to a person's health, that these Leal's would be selected against that this disease would become very rare in the human population. And that's actually the case in many parts of the world, but notably in parts of the world where malaria, the mosquito borne disease, is prevalent. It's actually the presence of this. Alil is much more common, and that's because of the hetero zegas advantage. It turns out that people who are home a zegas dominant have a disadvantage to those who are hetero zegas. So hetero zygotes have both copies of the Lille, and so their blood cells look like this. They have a mix of the sickle cells and normal cells, so It turns out that these individuals fare better with malaria because the sickle cells give them a sort of natural defense to the disease. Whereas home, a zegas dominant individuals do not have that same advantage, and they're far more susceptible to the disease malaria. So that is the hetero zegas advantage, illustrated through sickle cell anemia. Now balancing selection can also occur due to frequency dependent selection, which is when certain Leal's are favored Onley when they are rare, meaning that certain wheels are favored when they are uncommon in the population. Unexamined of this is the wheels for coloration in guppies. Yeah, I know strange example, but particular Leal's that give guppies a rare coloration. A color that's not common in the population gives them an advantage when it comes to predation. You see, predators tend to learn to recognize the familiar shapes and colors of their prey, so guppies that have the common coloration the common Lille in the population are not favored, but the wheels that give rare colorations those air favored. However, if those a leal's for rare coloration become mawr common in the population, then they become less favored, so frequency dependent selection is when certainly eels are favored when they are rare and Onley when they are rare. Ecological selection is natural selection without any sexual selection. So essentially it's natural selection. It's when Onley ecological influences affect selection because sexual selection is out of the equation, meaning that we have relatively random mating, meaning that Onley ecological factors are going to affect the natural selection process. So gene flow is something we talked about Hardy Weinberg model. And this is the transfer of a Leal's from one population to another, potentially altering gene frequencies. That's the important part. You have a nice example of that here to populations of the same species of birds that are geographically separated by this mountain. But you have a little gene flow. You have one of these red birds, uh, mate with someone from pop the Bluebird population and sneak, it's alil into that population. It's little h alil. Into that population, you have one of these bluebirds made with red Bird and sneak its capital H alil into the red bird population, and that is gene flow, the transfer of wheels from one population to another. Now let's turn the page and talk about some other influences on genetic variation

genetic drift is another way that we can get changes in Lille frequencies, and it's due to what's called random sampling. But basically the idea is that some a leal's get lucky in terms of the numbers. So we looked at when we talked about Mandelli in genetics. We looked at the, uh, the probabilities of Lille distributions from parents to offspring, and we saw that probability dictates that certain Leal's will be distributed at certain frequencies. But but that doesn't always happen. Just because there is a probability that it will occur doesn't necessarily mean that it will occur. And that's where randomness comes into play. So you can see here that we have a nice 50 50 mix of blue and red marbles. But over the generations are red. Marbles are more or less disappearing due to random sampling, and that is how genetic drift works. Basically, some wheels get lucky and others get unlucky, and we have a shift in Lille frequencies because our illegal distributions are not following those probability rules. Now the founder effect is when a small group from the population splinters off and forms a new one, taking a random sample of a Leal's with it. So you see, here in our starting population there's a relatively even number of of green and red dots. But our founding population doesn't have that same distribution. And in fact, over time we see that our founding population winds up with different alil and genotype frequencies than our original population. And that's due to the founder effect. The fact that these guys all left the group taking a random sample of Leal's that was not necessarily reflective of this larger populations, Alil frequencies and therefore their new population will show different illegal frequencies. With last type of genetic variation we're going to talk about is the bottleneck effect. And this is when population size dramatically decreases due to some random event, and this drastically alters alil frequencies so you can see and you take myself out of the image. Here. You can see a nice chart of population size over time and right here that's where we have this bottleneck event, which causes our population to plummet, and either it's going to go extinct or recover, and assuming it recovers well, when it recovers, it might wind up having you might wind up with a population of equal size and totally different legal frequencies because of the bottle necking. So you can see here we have a wide distribution of a Leal's. We have yellow, red, blue, green and pink. But our bottle necking event leaves us with mostly red, little blue and a little green. We don't have any more pink or yellow in the mix anymore. And that means if our population recovers, the illegal frequencies are going to look quite different than the initial population. That's all I have for this video on genetic variation and selection. I'll see you guys next time.