Frequency-dependent selection


Frequency-dependent selection is a type of natural selection where the fitness of a phenotype depends on the frequency of that phenotype in the population. It comes in two flavours:

  • Negative frequency-dependent selection, where the fitness of a phenotype increases when its frequency in the population is more rare
  • Positive frequency-dependent selection, where the fitness of a phenotype increases when its frequency in the population is more abundant

An example of negative frequency-dependent selection might be in pathogenic bacteria or viruses. For example, when a group of viruses is exposed to a human population, the humans will gradually acquire immunity to the virus. Thus, a minority strain of viruses which are able to 'rise above' this immune response due to some mutation in their genome will be more successful in continuing to make progeny. Hence, the viral phenotype has increased fitness because it is a rarity in this 'population' of viruses.

Conversely, an example of positive frequency-dependent selection might be in the development of warning colouration, in plants or insects, to prevent predation by birds or mammals. Warning colouration is only effective when it is 'common knowledge' that the colouration symbolises that the plant or insect is poisonous. While the colouration is a rare phenotype, it is unrecognised by predators and the plant or insect will be eaten regardless. Hence, the colouration phenotype requires an abundant frequency in the population in order for it to have increased fitness.

In both cases, the phenotype does not inherently bring fitness to the organism. It is only in the context of a certain phenotypic frequency, low or high, that the phenotype can be considered to have fitness.