American Society of Naturalists

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“Assortative mating in hybrid zones is remarkably ineffective in promoting speciation”

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Darren E. Irwin (June 2020)

What happens when 2 pops meet with strong assortative mating & no hybrid problems? Simulations show they blend together!

Read the Article (Just Accepted)

Can assortative mating really cause speciation?

An example of a narrow hybrid zone that is likely maintained by slightly reduced fitness of hybrids is the Townsend’s&nbsp;/ Hermit warbler hybrid zone (<i>Setophaga townsendi</i> / <i>S.&nbsp;occidentalis</i>). Here, a male hybrid sings in the middle of the zone, near Mt. Rainier, Washington.  <br />(Credit: Darren E. Irwin)
An example of a narrow hybrid zone that is likely maintained by slightly reduced fitness of hybrids is the Townsend’s / Hermit warbler hybrid zone (Setophaga townsendi / S. occidentalis). Here, a male hybrid sings in the middle of the zone, near Mt. Rainier, Washington.
(Credit: Darren E. Irwin)

Speciation, or the evolution of two species from one, is often said to start with the evolution of assortative mating, the tendency of individuals to choose mates similar to themselves. In this line of thinking, assortative mating between two populations is a form of “premating reproductive isolation” of two groups. This then allows the two groups to become more different over time, such that low hybrid fitness (“postzygotic reproductive isolation”) eventually develops between them. What happens if we use simulations to formally test these ideas? Professor Darren Irwin of the University of British Columbia built computer models of two populations coming into contact, with varying levels of assortative mating and hybrid fitness. To his initial surprise, even quite strong assortative mating (e.g. a 10 times greater mating preference for an individual’s own population) has little impact on limiting the formation of a broad hybrid zone. This is because rare hybrids can mate with each other, leading to growing numbers of hybrids and backcrosses, eventually forming a continuous genetic bridge between the populations. In contrast, a small (e.g. 5%) reduction in the fitness of hybrids has a large impact on limiting the width of the zone. These results call into question the concept of partial premating reproductive isolation, as it is ineffective if there is not also reduced fitness of hybrids. These findings also encourage speciation researchers to renew their interest in examining the fitness of hybrids, as that is the crucial determinant of whether two populations blend together or continue to evolve into two species.


Abstract

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.