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.

• In other butterflies, such as in central and south American Heliconius, there are several morphs within a species, each morph having a different color pattern. All the morphs are poisonous. When a morph is common, it will be more likely that birds will have already learned to avoid them, whereas birds will not yet have learned to avoid a rare morph. An individual of a rare morph is therefore more likely to be the unlucky prey that educates the bird, and gets killed in the process. The fitness of each morph is positively frequency-dependent.

But with negatively frequency-dependent fitnesses (as in Batesian mimicry), it is possible for natural selection to maintain a polymorphism. When a genotype is rare, it is relatively favored by selection and it will increase in frequency; as it becomes more common, its fitness decreases and there may come a point at which it is no longer favored. At that point, the fitnesses of the different genotypes are equal and natural selection will not alter their frequencies: they are at equilibrium. The sex ratio is another case in which selection is frequency-dependent.

The image opposite shows different forms of the species Heliconius erato.