Frequency dependent selection
Frequency dependent selection (or Dynamic 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. 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.
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.
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.