In Directional Selection, the average phenotype of a population shifts in one direction, usually favoring an extreme phenotype. Imagine that most birds in a particular population were sort of a midsize between a small bird and a big bird. 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 these big birds. It can be pushed in either direction depending on the pressure from the environment.
You also will sometimes see Stabilizing Selection, and this is when extreme phenotypes, like the really small bird and the really big bird, those extreme phenotypes are selected against. So, actually, the average phenotype doesn't change. The average phenotype stays the same. However, genetic variation is reduced. We go from this outer red curve to this inner blue one. The average, this middle-of-the-road phenotype stays the same, but the percentage of the population that has more extreme phenotypes reduces.
You also will sometimes see stabilizing selection due to a purifying or negative selection, and this is the removal of deleterious alleles or alleles that have harmful effects. Purifying selection doesn't always lead to stabilizing selection, but it can, and that's usually when those deleterious alleles result in some extreme phenotype.
Lastly, you can actually have disruptive selection, and that's when the extreme phenotypes are favored over the intermediate ones. But in this case, both extremes are favored. Unlike Directional Selection where we push in one direction or the other, here in disruptive selection, we actually go for both extremes. We start off with this red curve and wind up with this blue curve in terms of the distribution of phenotypes. Here the midsize birds are being selected against, and small and large birds are being favored.
Now, let's turn the page and talk about some more types of natural selection.