Assortative mating in hybrid zones is remarkably ineffective in promoting speciation

Abstract

Running title: "Assortative mating and hybrid zone width" Data Accessibility: Upon acceptance for publication in a peer-reviewed journal, data and code will be deposited in the Dryad Digital Repository. AbstractAssortative mating and other forms of partial prezygotic isolation are often viewed as being more important than partial postzygotic isolation (low fitness of hybrids) early in the process of speciation. Here we simulate secondary contact between two populations ('species') to examine effects of pre-and postzygotic isolation in preventing blending. A small reduction in hybrid fitness (e.g., 10%) produces a narrower hybrid zone than a strong but imperfect mating preference (e.g., 10x stronger preference for conspecific over heterospecific mates). This is because, in the latter case, rare F1 hybrids find each other attractive (due to assortative mating), leading to the gradual buildup of a full continuum of intermediates between the two species. The cline is narrower than would result from purely neutral diffusion over the same number of generations, but this effect is due to the frequency-dependent mating disadvantage of individuals of rare mating types. Hybrids tend to pay this cost of rarity more than pure individuals, meaning there is an induced postzygotic isolation effect of assortative mating.When this induced mating disadvantage is removed, partial assortative mating does not prevent eventual blending of the species. These results prompt a questioning of the concept of partial prezygotic isolation, since it is not very isolating unless there is also postzygotic isolation.

Assortative mating, sexual selection and their consequences for gene flow inLittorina

“…Our results broadly agree with Irwin's (2019) conclusion that assortative mating adds rather little to the barrier effect created by natural selection in a hybrid zone. Because we considered a multiple-effect trait, whereas Irwin considered a signal-preference interaction that was separate from the trait under natural selection, we might have expected a stronger effect.…”

“…In turn, these isolation indices come from experiments where individuals can mate either within their own population or with an individual from a divergent population (e.g., Matsubayashi and Katakura 2009). However, these indices risk over-simplifying the mating pattern and they fail to account for the presence of the intermediate phenotypes that are present whenever reproductive isolation is incomplete (Coyne and Orr 2004;Irwin 2019).…”

“…This provides an excellent opportunity to quantify the barrier effects of assortative mating and sexual selection. It has been argued that assortative mating based on clinally-varying traits will generate only a weak barrier to gene flow because individuals that meet one-another in the hybrid zone rarely differ much in trait values, allowing little opportunity for discrimination (Irwin 2019).…”