Modes of Selection - Background
There are three general modes of selection:
Directional Selection occurs when selection favors one extreme trait value over the other extreme. This typically results in a change in the mean value of the trait under selection.
Disruptive Selection occurs when selection favors the extreme trait values over the intermediate trait values. In this case the variance increases as the population is divided into two distinct groups. Disruptive selection plays an important role in speciation.
Stabilizing Selection occurs when selection favors the intermediate trait value over the extreme values. Populations under this type of selection typically experience an initial decrease in the amount of additive genetic variation for the trait under selection. However, stabilizing selection may be responsible for long-term maintenance of genetic variation.