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. Stabilizing selection is a potential model for fixation of a trait, such as seven cervical vertebrae in Mammalia [count them in the Blue Whale], or four wings in Insecta.

    Divergent selection favors the upper and (or) 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 modes. Especially when disruptive selection occurs at loci involved in mate choice, it is a potential model for Speciation.

    Directional selection favors one tail over both the modal values and the other tail: the mean shifts unidirectionally, and the variance remains unchanged. Directional selection is a potential model for Darwinian Evolution, as Descent with Modification.