On the origin of species by means of assortative mating

Assortative mating may split a population even in the absence of natural se lection. Here, we study when this happens if mating depends on one or two q uantitative trails. Not surprisingly, the modes of assortative mating that can cause sympatric speciation without selection are rather strict. However , some of them may occur in nature. Slow elimination of intermediate indivi duals caused by the gradual tightening of assortative mating, which evolves owing to relatively weak disruptive selection, provides the alternative sc enario for sympatric speciation, in addition to fast elimination of interme diate individuals as a result of the direct action of strong disruptive sel ection under an invariant mode of assortative mating. Even when assortative mating alone cannot split an initially coherent population, it may be able to prevent the merging of species after their secondary contact.