VIII.4 Type II polymorphism can be maintained in the population only by specific mechanisms

The existence of Type I polymorphism can be readily understood and follows from the very existence of mutation processes and from the existence and properties of the individual evolutionary processes, such as natural selection andgenetic drift.The existence of Type II polymorphism is a quite unexpected phenomenon that warrants a separate explanation.If we assume that all organisms are exposed to the systematic action of natural selection, it could be expected to a first approximation that various alleles of a single gene would not be able to survive for long times in the population.It is very probable that all the alleles would have the same selection value and that they would be completely equivalent from the standpoint of the evolutionary fitness of the organism.Consequently, it could be expected that the most valuable allele would gradually displace less advantageous alleles.

Evolutionary biologists have described a number of mechanisms that can prevent this displacement and that can thus enable long-term survival of polymorphism of Type II in populations.They include combination of mutation drive with recessiveness of traits, superdominance, frequency-dependent selection, cyclic selection and the effect of epistatic interactions.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.