The main subject for models of postzygotic isolation has been how reproductive isolation genes (RI genes) which cause hybrid inviability or sterility spread within populations despite their deleterious effects. The models are divided into three categories according to the within-population effect of RI genes in their fixation process. (1) The beneficial effect model, where RI genes are assumed to spread within populations by a positive selective force via natural or sexual selection. (2) The neutral effect model, where RI genes are assumed not to affect the fitness of individuals in their fixation process and to be spread by genetic drift. (3) The deleterious effect model, where RI genes are assumed to exhibit some (slightly) deleterious effects in their fixation process and to be spread by genetic drift. Factors that affect the applicability of these models are discussed. If a selective force such as sexual conflict or natural selection facilitates the evolution of RI genes, the beneficial effect model should be applied. Many empirical studies have suggested that positive selection plays an important role in the evolution of hybrid male sterility. If the mutation rates of RI genes are low, and the specificity of epistatic interaction causing hybrid inviability or sterility is high, the neutral effect model should be applied. However, if the opposite condition applies, the deleterious effect model should be applied.
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