Frequency-dependent selection
![Anvil stone, where a thrush has broken open the shells of polymorphic grove snails, may be a sign of frequency-dependent selection.](/Images/godic/202501/14/The_anvil_stone_of_a_thrush_-_geograph.org.uk_-_17513480736.jpg")
![Venomous coral snake's warning coloration can benefit harmless mimics, depending on their relative frequency.](/Images/godic/202501/14/Coral_snake0736.jpg")
![Harmless scarlet kingsnake mimics the coral snake, but its pattern varies less where the coral snake is rare.](/Images/godic/202501/14/G-Bartolotti_SK0736.jpg")
Frequency-dependent selection is the term given to an evolutionary process where the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population.
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 αij parameters in Lotka-Volterra competition equations are non-zero).