Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

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.