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 (or purifying frequency-dependent selection) the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection (or diversifying frequency-dependent selection) the fitness of a phenotype increases as it becomes rarer. Negative frequency-dependent selection is an example of balancing selection. 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 α_ij parameters in Lotka-Volterra competition equations are non-zero).

Negative frequency-dependent selection

The first explicit statement of frequency dependent selection appears to have been by E.B. Poulton in 1884 with reference to the way that predators could maintain color polymorphisms in their prey.^[1][2] Perhaps the best known early modern statement of the principle in the twentieth century was the discussion by Bryan Clarke of apostatic selection (a synonym of negative frequency-dependent selection).^[3] In this paper, Clarke discussed frequency-dependent selection particularly with regard to predator attacks on polymorphic British snails, citing Luuk Tinbergen's classic work on searching images as support that predators such as birds would tend to specialize on common forms of palatable species.^[4] Clarke later became a major promoter of the idea that frequency-dependent balancing selection might explain abundant molecular polymorphisms (often in the absence of heterosis) in opposition to the neutral theory of molecular evolution.

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^{[citation needed]}.

Negative frequency-dependent selection also operates in the interaction of many human pathogens, such as the flu virus^{[citation needed]}. 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 ^[5]). 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. In the between-species analogue, this is equivalent to an Allee effect, in which if a species is too rare, it may decline to extinction. 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, is known as Müllerian mimicry, a form of convergent evolution where multiple species share the same advantageous pattern.

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 1884: 27–60.
2. ^ Allen, J.A., and B.C. Clarke. 1984. Frequency-dependent selection -- homage to Poulton, E.B. Biological Journal of the Linnean Society 23:15-18.
3. ^ Clarke, B. 1962. Balanced polymorphism and the diversity of sympatric species. Pp. 47-70 in D. Nichols ed. Taxonomy and Geography. Systematics Association, Oxford.
4. ^ Tinbergen, L. 1960. The natural control of insects in pinewoods. I. Factors influencing the intensity of predation in songbirds. Archs.Neerl.Zool. 13:265-343.
5. ^ Takahata, N., and M. Nei. 1990. "Allelic genealogy under overdominant and frequency-dependent selection and polymorphism of major histocompatibility complex loci". Genetics 124: 967–78.

Frequency-dependent selection is one in which the fitness of genotypes change according to their frequencies in population (Tamarin: Principles of Genetics;7th edition;2001).

See also

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