Frequency-dependent selection

Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population Frequency-dependent selection is an evolutionary process by which 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 [[Polymorphism ..(biology)|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). The first explicit statement of frequency-dependent selection appears to have been by Edward Bagnall Poulton in 1884, on the way that predators could maintain color polymorphisms in their prey. Perhaps the best known early modern statement of the principle is Bryan Clarke's 1962 paper on apostatic selection (a synonym of negative frequency-dependent selection). Clarke discussed predator attacks on polymorphic British snails, citing Luuk Tinbergen's classic work on searching images as support that predators such as birds tended to specialize in common forms of palatable species. Clarke later argued that frequency-dependent balancing selection could explain molecular polymorphisms (often in the absence of heterosis) in opposition to the neutral theory of molecular evolution. Another example is plant self-incompatibility alleles. When two plants share the same incompatibility allele, they are unable to mate. Thus, a plant with a new (and therefore, rare) allele has more success at mating, and its allele spreads quickly through the population. In human pathogens, such as the flu virus, once a particular strain has become common, most individuals have developed an immune response to that strain. But a rare, novel strain of the flu virus is able to spread quickly to almost any individual, causing continual evolution of viral strains. The major histocompatibility complex (MHC) is involved in the recognition of foreign antigens and cells. Frequency-dependent selection may explain the high degree of polymorphism in the MHC. In behavioral ecology, negative frequency-dependent selection often maintains multiple behavioral strategies within a species. A classic example is the Hawk-Dove model of interactions among individuals in a population. In a population with two traits A and B, being one form is better when most members are the other form. As another example, male common side-blotched lizards have three morphs, which either defend large territories and maintain large harems of females, defend smaller territories and keep one female, or mimic females in order to sneak matings from the other two morphs. These three morphs participate in a rock paper scissors sort of interaction such that no one morph completely outcompetes the other two. Another example occurs in the scaly-breasted munia, where certain individuals become scroungers and others become producers. Positive frequency-dependent selection gives an advantage to common phenotypes. A good example is warning coloration in aposematic species. Predators are more likely to remember a common color pattern that they have already encountered frequently than one that is rare. This means that new mutants or migrants that have color patterns other than the common type are eliminated from the population by differential predation. Positive frequency-dependent selection provides the basis for Müllerian mimicry, as described by Fritz Müller , because all species involved are aposematic and share the benefit of a common, honest signal to potential predators.