Assortative mating in hybrid zones is remarkably ineffective in promoting speciation

Partial prezygotic isolation is often viewed as more important than
partial postzygotic isolation (low fitness of hybrids) early in the
process of speciation. I simulate secondary contact between two
populations (‘species’) to examine effects of assortative mating and low
hybrid fitness in preventing blending. A small reduction in hybrid fitness
(e.g., by 10%) produces a narrower hybrid zone than a strong but imperfect
mating preference (e.g., 10x stronger preference for conspecific over
heterospecific mates). In the latter case, rare F1 hybrids find each other
attractive (due to assortative mating), leading to the buildup of a
continuum of intermediates. The weakness of assortative mating compared to
reduced fitness of hybrids in preventing blending is robust to varying
genetic bases of these traits. Assortative mating is most powerful in
limiting blending when it is encoded by a single locus, is essentially
complete, or when there is a large mate search cost. In these cases
assortative mating is likely to cause hybrids to have low fitness, due to
frequency-dependent mating disadvantage of individuals of rare mating
types. These results prompt a questioning of the concept of partial
prezygotic isolation, since it is not very isolating unless there is also
postzygotic isolation.