Frequency dependent selection

From Freepedia

Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on the relative frequency of 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 decreases as it becomes more common. Frequency dependent selection is a particular mechanism of balancing selection.

One 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.

Another relevant example in humans is lefthandedness: in a righthanded world, lefthanded people have the edge in duel-type combat (like in the modern world in tennis). This advantage diminishes as lefthandedness becomes more common. Thus, the percentage of lefthanded people has settled on an optimum of around 5%.

An example of positive frequency dependent selection would be the behavioral phenotype of favoring or functioning better in communities. If few organisms favor communities, then few communities will exist and the organisms would be at a disadvantage. As the number of community-minded organisms increases, however, the community phenotype becomes more fit.