NATURAL SELECTION
The force of natural selection tends to reduce the genetic variability of populations in a way that increases adaptation. It does so by removing or reducing the frequency of some phenotypes and increasing the frequency of others.

Three kinds of selection cause changes in the normal distribution of phenotypes in a population. Stabilizing selection eliminates those phenotypes most different from the norm, thus reducing the frequency of phenotypic extremes. Directional selection eliminates one extreme and moves the population toward the other. Disruptive selection eliminates average phenotypes and encourages the extremes. This tends to result in distinct phenotypes in the same population.