Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population. Frequency-dependent selection occurs when the fitness of a genotype depends on its frequency.
It is possible for the fitness of a genotype to increase (positively frequency-dependent) or decrease (negatively frequency-dependent) as the genotype frequency in the population increases.
Examples of frequency dependence can arise in systems of mimicry:
• Natural selection may favor non-poisonous butterflies that have the same color pattern as poisonous butterflies. This system is called Batesian mimicry. When they are rare, birds will tend to avoid the mimics, because they will have already have encountered a poisonous butterfly of the same appearance. But when the non-poisonous type is common, the previous encounters of birds with butterflies of their appearance are more likely to have been rewarding; the birds will not avoid eating them, and their fitness will be lower. The fitness of the mimics is negatively frequency-dependent.