What type of selection favors extreme phenotypes over intermediate?

The type of selection that favors extreme phenotypes over intermediate ones is disruptive selection. It is also called diversifying selection because it can push a population toward two or more distinct trait values rather than toward an average.

For example, imagine birds feeding on either very large or very small seeds, while medium-sized seeds are scarce. Birds with very large or very small beaks may obtain food more effectively than birds with intermediate beaks. If those differences affect survival or reproduction, both extremes can become more common and the intermediate form can decline.

Disruptive selection differs from directional selection, which favors one extreme, and stabilizing selection, which favors intermediate phenotypes and selects against both extremes. Sexual selection is a separate category involving differences in mating success, although sexual selection can sometimes produce extreme traits. By increasing variation and potentially separating populations into different ecological niches, disruptive selection may contribute to divergence and, in some circumstances, speciation. It does not automatically create new species, however; geographic isolation, reproductive barriers, and other evolutionary conditions may also be required.