Directional selection

In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction. Under directional selection, the advantageous allele increases in frequency independently of its dominance relative to other alleles; that is, even if the advantageous allele is recessive, it will eventually become fixed. Directional selection stands in contrast to balancing selection where selection may favor multiple alleles, and is the same as purifying selection which removes deleterious mutations from a population.

Directional selection occurs most often under environmental changes and when populations migrate to new areas with different environmental pressures. An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches. Throughout the wet seasons, small seeds were more common and there was such a large supply of the small seeds that the finches rarely ate large seeds. During the dry seasons, none of the seeds were in great abundance, but the birds usually ate more large seeds. The change in diet of the finches affected the depth of the birds’ beaks in the future generations.Their beaks range from large and tough to small and smooth.