Three modes of
Natural Selection
Stabilizing selection favors the modal
phenotypic trait values, by selecting against the
upper and lower tails of the distribution. The mean remains
the same, while the variance decreases. Divergent
selection
favors the upper and lower tails, by selecting against
the modal values; the mean remains the same, while the
variance increases. In the extreme case of Disruptive
selection, the distribution may become bimodal,
with two means. Directional selection favors one
tail over both the modal values and the other tail: the mean
shifts unidirectionally, and the variance remains
unchanged.