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Difference in fitness of alleles or genotypes as a function of their prevalence within populations.
The standard situation is that the reproductive potential of a given allele or genotype is independent of how prevalent that allele is in a population (that is, the per capita reproductive success does not vary, e.g., if 1% versus 99% of a population possess that allele or genotype). With frequency-dependent selection, however, reproductive potential does vary with prevalence of a type.
That fitness can decrease with increasing prevalence, which can be described as a stabilizing frequency-dependent selection. Alterantively, fitness can increase with increasing prevalence, which can be described as a disruptive frequency-dependent selection. In either case, what is or is not being stabilized are balanced polymorphisms.
Frequency-dependent selection can be viewed as an additional layer of complexity – along with epistasis, tradeoffs, pleiotropy, genetic drift, etc. – that serves to modify the impact of natural selection on populations.
See also the related concepts of allele frequency and genotype frequency.
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