Frequency-dependent selection

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.

• In positive frequency-dependent selection, the fitness of a phenotype or genotype increases as it becomes more common.
• In negative frequency-dependent selection, the fitness of a phenotype or genotype decreases as it becomes more common. This is an example of balancing selection.
• More generally, frequency-dependent selection includes when biological interactions make an individual's fitness depend on the frequencies of other phenotypes or genotypes in the population.^[1]

Frequency-dependent selection is usually the result of interactions between species (predation, parasitism, or competition), or between genotypes within species (usually competitive or symbiotic), and has been especially frequently discussed with relation to anti-predator adaptations. Frequency-dependent selection can lead to polymorphic equilibria, which result from interactions among genotypes within species, in the same way that multi-species equilibria require interactions between species in competition (e.g. where α_ij parameters in Lotka-Volterra competition equations are non-zero). Frequency-dependent selection can also lead to dynamical chaos when some individuals' fitnesses become very low at intermediate allele frequencies.^[2][3]