Directional, Stabilizing, and Disruptive Seleciton

by Jason Amores Sumpter
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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