Sympatric Speciation

It is shown that in a heterogeneous environment divided into two "niches," a stable polymorphism can exist between two alleles each conferring a selective advantage in one of the niches, even if adults form a single random-mating population, provided: (1) the population size is separately regulated in the two niches, and (2) the selective advantages are large. The conditions to be satisfied are given; it is difficult to say how often they will be satisfied. A stable polymorphism is favored if females tend to lay eggs in the niche in which they were themselves raised. Four genetic mechanisms--habitat selection, pleiotropic genes, modifying genes, and assortative mating genes--which could cause mating isolation are considered. It is concluded that stable polymorphism could be the first stage in sympatric speciation. The relevance of these mechanisms in allopatric speciation is also discussed.