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
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