Frequency dependent selection

Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. In positive frequency dependent selection, the fitness of a phenotype increases as it becomes more common. In negative frequency dependent selection, the fitness of a phenotype increases as it becomes less common. Negative frequency dependent selection is a particular mechanism of balancing selection.

Negative frequency dependent selection

Negative frequency dependent selection is often the result of interactions between species (predation, parasitism,or competition) or between genotypes within species (usually competitive or symbiotic). The first explicit statement of frequency dependence appears to have been by E.B. Poulton in 1884 with reference to the way that predators could maintain color polymorphisms in their prey.

Another example of negative frequency dependent selection is in the case of 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.

Negative frequency dependent selection also operates in the interaction of many human pathogens, such as the flu virus. Once a particular strain has been common in a human population, most individuals would have developed an immune response to that strain. But a rare, novel strain of the flu virus would be able to spread quickly to almost any individual. This advantage of genetic novelty causes continual evolution of viral strains, with new versions common each year. Another immune-related example of negative frequency dependent selection is the major histocompatibility complex, which is involved in recognition of foreign antigens (Takahata and Nei, 1990). Frequency dependent selection as an important factor may explain the high degree of polymorphism of MHC (Borghans et al., 2004)^{[citation needed]}.

Positive frequency dependent selection

Where negative frequency dependent selection gives an advantage to rare phenotypes, positive frequency dependent selection gives an advantage to common phenotypes. This means that new alleles can have a difficult time invading a population, since they don't experience significant benefit until they become common. This has been proposed as a difficulty in the evolution of aposematic (or warning) coloration in toxic or distasteful organisms. The presumed advantage of the aposematic coloration is that predators have learned to avoid potential prey with that color pattern. But when the pattern is rare, the predator population does not become 'educated' and the pattern has no benefit. Therefore the warning color is only advantageous once it has become common. Warning coloration, if it involves more than one species, may also be an example of convergent evolution.

References

1. Poulton, E. B. (1884). "Notes upon, or suggested by, the colours, markings and protective attitudes of certain lepidopterous larvae and pupae, and of a phytophagous hymenopterous larva". Transactions of the Entomological Society of London: 27–60. 
2. Takahata, N; M Nei (1990). "Allelic genealogy under overdominant and frequency-dependent selection and polymorphism of major histocompatibility complex loci". Genetics 124 (4): 967–78. ISSN 0016-6731. PMID 2323559. 

See also

• Evolutionary game theory
• Evolutionarily stable strategy
• Mimicry
• Tit for tat
• Apostatic selection