A population of finches experiences a drought where all of the normal food types disappear leaving only ant hills that certain finches with thin beaks can access. Over time, almost all of the population has a similar long sharp beak. This is an example of Directional selection.
Directional selection occurs when individuals homozygous for one allele have a fitness greater than that of individuals with other genotypes and individuals homozygous for the other allele have a fitness less than that of individuals with other genotypes.
At equilibrium the population will be composed entirely of individuals that are homozygous for the allele associated with the highest probability of survival. The rate at which the population approaches this equilibrium depends on whether the favored allele is dominant, partially dominant, or recessive with respect to survival probability.
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