Directional selection

In population genetics, directional selection occurs when natural selection favors a single allele and therefore allele frequency continuously shift in one direction. It is in contradistinction to balancing selection where selection may favor multiple alleles. Directional selection is a particular mode or mechanism of natural selection.

Example

A common example is the peppered moth (Biston betularia). Before the industrial revolution in England (1740?), the peppered moth was mostly found in a light gray form with little black speckled spots. The allele for dark-bodied moths is dominant, while the allele for light-bodied moths is recessive. The light-bodied moths were able to blend in with the light colored lichens and