IV.5 If the fitness of bearers of a certain allele depends on their incidence in the population, this is called frequency-dependent selection.
The biological fitness of an individual is frequently determined, not only by his phenotype, but also by the phenotypes of the other members of the population.For example, in any form of soft selection, the chances of survival of an individual depend not on the absolute value of his traits, but on the degree to which and the direction in which his traits differ from the traits of an average member of the given population.
However, more complicated cases also frequently occur, where the fitness of an individual changes stepwise in dependence on the frequency of his allele in the remaining population, even under the conditions of hard selection. An example is the situation in which a population of prey, exposed to the activities of a certain predator, finds itself.It is known that a predator will frequently select the commonest type of prey as a target in a particular environment (Amalraj & Das 1996; Allen 1988; Gotmark & Olsson 1997).If the prey occurs in two different colour forms, determined, e.g., by a pair of alleles, then the predator will always concentrate on the bearer of the more common allele.Thus, the frequency of this allele will decrease in the population as a consequence of “apostatic selection” (Allen, Raison, & Weale 1998)until the bearers of the alternative allele predominate in the population.Then, the predator will concentrate on the bearers of the alternative allele, so that the fitness of the individuals with the originally frequent allele (now rare) will suddenly increase.
Frequency-dependent selectionoccurs, e.g., in some types of sexual selection.In some species of organisms, the rare-male advantage phenomenon is active. Here females mate preferentially with the bearers of rare traits, i.e. the bearers of rare alleles (Dernoncourt-Sterpin, Leichien, & Elens 1991; Depiereux et al. 1990).Because of the preference for these males, the frequency of the rare alleles increases and thus other alleles become rare, i.e. advantageous.On the other hand, in some species, females can prefer the bearers of the most frequent alleles; in this case, we once again speak of frequency-dependent selection; however the less frequent alleles then rapidly disappear from the population.
Frequency-dependent selection acting in favour of less frequent alleles is probably one of the most important mechanisms for long-term maintenance of polymorphism in the population (Antonovics & Ellstrand 1984; Elena & Lenski 1997; Benkman 1996).As this type of selection can occur not only within populations and within species, but is also a matter of interspecies competition (a predator can select the members of the commonest species), frequency-dependent selection can create the preconditions for the long-term co-existence of two various species at a single location.