disruptive selection

Meaning: Disruptive (diversifying) selection is natural selection in which individuals at both extremes of a trait have higher fitness than intermediates. A single-peaked distribution becomes bimodal. It tends to occur when an environment offers two distinct niches and intermediates do poorly in both, such as small beaks suited to soft seeds and large beaks suited to hard seeds, with medium beaks efficient at neither. If extreme types mate mostly with their own kind, gene flow between them falls, which can contribute to speciation, though it does not always lead to it.

On the MCAT: A passage gives a histogram or curve of a trait before and after selection. Two peaks with a dip in the middle means disruptive selection; one narrower peak means stabilizing; one peak that shifts means directional. Expect questions on variance, which increases here, and on the mean, which may stay about the same.

Common confusion: Students mistake it for directional selection because both favor an extreme. Disruptive selection favors both extremes and acts against the middle, so the mean may not move while variance rises.