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Frequency Dependent Selection

M.Tevfik Dorak, M.D., Ph.D.

An evolutionary process where the fitness of a phenotype is dependent on the relative frequency of other phenotypes in the population is called frequency dependent selection. 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. In other words, less frequent phenotypes have higher fitnesses than common ones.

Haldane was the first to recognize that host-parasite relationships can lead to a cyclical coevolutionary arms race at the molecular level. Natural selection would favor those parasite genotypes that can best evade host resistance mechanisms of the most common host genotypes, because they would be capable of persisting in a large proportion of the host population. This disproportionate parasite burden would then lower the fitness of individuals bearing the common allele, thereby favoring rare alleles. As rare alleles increase in frequency the whole process is repeated leading to a stable polymorphism of host and parasite genotypes, with newly arising host alleles often being favored because pathogens have not had tome to adapt to these new variants. Thus, rare alleles are favored when infrequent and disfavored as they become common. As this process would not allow fixation of an allele, a stable polymorphism is maintained. Frequency dependent selection is a dynamic process while the other form of balancing selection -heterozygous advantage- is a stable one (see below).

There are several examples of (negative) frequency dependent selection:

1. Maintenance of a 50:50 sex ratio: If one sex becomes more common, some of its members will not be able to mate whereas al members of the less frequent sex will mate. This will result in higher fitness for the rare sex and the sex ratio will return to the balance.

2. Maintenance of polymorphism in prey species: Predators commonly prefer the commoner type of a prey. A prey type that is rare will thus tend to be at an advantage because it suffers lower predation than commoner prey types. This maintains the polymorphism among preys.

3. The rare male effect: Females of some polymorphic species prefer to mate with males who belong to a rarer phenotype. When this type gets more common, however, its fitness decreases as the other (now rarer) type will be preferred. Thus both types remain in the population.

4. Competition among males for females (the caller-satellite system of green frogs): Frogs choose the mating strategy (being a caller or satellite) whichever seems to be more advantageous each type. The mating success of each type depends on the frequency of the type relative to the frequency of the other type (if they are all callers or all satellites the mating success will be very low for all of them).

5. Competition for resources: Similar to the above example, the relative frequencies of alternative competitive strategies may be determined by frequency dependent selection. The more common one type becomes, the lower is the average fitness of individuals of that type and the higher is the average fitness of an alternative type.

6. In prey species mimicking a poisonous species (Batesian mimicry), this is only advantageous if the mimic is less common than the model. In other words, the model should be much commoner. The fitness of the mimics is negatively frequency dependent. If their frequency increases and especially is exceeds to that of the model, the advantage disappears.

Another form of selection, overdominance or heterozygous advantage where heterozygous genotype is selected over either of the homozygous genotypes, also has a (negative) frequency dependent component because rare alleles are disproportionately found in heterozygous genotypes, whereas common alleles are disproportionately found in homozygous genotypes. Thus rare alleles always appear to have an advantage. Heterozygous advantage results in a stable polymorphism, hence the alternative name balancing selection.

An interesting article suggesting frequency-dependent selection of Left-Handedness in Humans

M.Tevfik Dorak, MD, PhD

Last updated 9 January 2007

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