In evolutionary biology, what type of natural selection favors one extreme version of a trait over others?

Directional selection is the type of natural selection that favors one extreme version of a trait over others.

As a result, the population’s average value for that trait shifts toward the favored extreme. For example, if larger beaks help birds obtain available food, birds with larger beaks may leave more offspring, causing average beak size to increase over generations.

Directional selection differs from stabilizing selection, which favors intermediate traits, and disruptive selection, which favors both extremes. It can be especially noticeable when environmental conditions change, such as during a drought or after the arrival of a new predator.

The favored direction is not permanent or universal: if the environment changes again, another trait value may become advantageous. Directional selection describes a pattern of evolutionary change, not a conscious goal or a guarantee that organisms become more complex.