Frequency-dependent selection

Frequency-dependent selection is an evolutionary mechanism in which the fitness of a genotype depends on its relative frequency in the population. Rare genotypes have higher fitness because common enemies — parasites, predators, or competitors — have not yet adapted to them; common genotypes suffer because their enemies have evolved countermeasures. This creates a perpetual oscillation in genotype frequencies that prevents any single variant from dominating the population, maintaining the genetic diversity that fuels antagonistic coevolution and the Red Queen dynamic. The mechanism is not limited to biology; it appears in any system where rarity confers advantage, including market strategies, fashion cycles, and immune system polymorphism. See Negative frequency-dependent selection for the mathematical formalization and balancing selection for the broader class of mechanisms that maintain polymorphism.