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Balancing selection
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=== Frequency-dependent selection === {{main|Frequency-dependent selection}} Frequency-dependent selection occurs when 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 decreases as it becomes more common. For example, in [[prey switching]], rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs. As predation drives the demographic frequencies of the common morph of prey down, the once rare morph of prey becomes the more common morph. Thus, the morph of advantage now is the morph of disadvantage. This may lead to boom and bust cycles of prey morphs. Host-parasite interactions may also drive negative frequency-dependent selection, in alignment with the Red Queen hypothesis. For example, parasitism of freshwater New Zealand snail (''Potamopyrgus antipodarum'') by the trematode ''Microphallus sp.'' results in decreasing frequencies of the most commonly hosted genotypes across several generations. The more common a genotype became in a generation, the more vulnerable to parasitism by ''Microphallus sp.'' it became.<ref>Koskella, B. and Lively, C. M. (2009), EVIDENCE FOR NEGATIVE FREQUENCY-DEPENDENT SELECTION DURING EXPERIMENTAL COEVOLUTION OF A FRESHWATER SNAIL AND A STERILIZING TREMATODE. Evolution, 63: 2213β2221. {{doi|10.1111/j.1558-5646.2009.00711.x}}</ref> Note that in these examples that no one phenotypic morph, nor one genotype is entirely extinguished from a population, nor is one phenotypic morph nor genotype selected for fixation. Thus, polymorphism is maintained by negative frequency-dependent selection.
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