Selection in one direction
Directional selection occurs when natural selection favours individuals towards one extreme of the range of phenotypes. Those individuals have greater reproductive success, so alleles associated with the favoured phenotype become increasingly common over successive generations.
This produces a progressive change in allele frequencies and shifts the population towards the favoured end of the distribution. As a result, the mean value of the characteristic changes over time.
Directional selection commonly follows:
- a change in the environment that alters the selection pressure; or
- the appearance of a new allele that is advantageous under the existing conditions.
Which end of the distribution is favoured depends on the phenotype that provides an advantage under the current selection pressure.