Directional selection is one of three main forms of natural selection. In directional selection, an extreme visible characteristic or trait (phenotype) is favored with regard to reproductive success. As with all forms of selection, directional selection is based upon differences in success in reproduction (differential reproduction) wherein organisms within a population that carry more advantageous genes are able to survive and produce more offspring (progeny) that other members of the population.

The other main mechanisms of natural selection include stabilizing selection (when an intermediate phenotype is fittest) and disruptive selection (extreme phenotypes are fitter than an intermediate phenotype).

Directional selection acts to produce genetic changes in populations. Accordingly, it is a prime evolutionary mechanism where there is also environmental change.

The peppered moth Biston betularia provided striking evidence of directional selection in post-industrial revolution England. In response to air pollution (primarily soot from coal burning industry...