Hybridization and the Coexistence of Species

Abstract

It is thought that two species can coexist if they use different resources present in the environment, yet this assumes that species are completely reproductively isolated. Closely related species often interbreed, raising the question of how this might affect coexistence. We model coexistence outcomes for two sympatric species that are ecologically differentiated but have incomplete reproductive isolation. Results show that the consequences of interbreeding depend crucially on hybrid fitness. When hybrid fitness is high, just a small rate of hybridization can lead to collapse of two species into one. Low hybrid fitness can cause population declines, making extinction of one or both species likely. The intrinsic growth rate of the population has an important influence on the outcome. High intrinsic growth rates result in higher reproductive rates when populations are below carrying capacity, reducing the probability of extinction and increasing the likelihood of stable coexistence at moderate levels of assortative mating and hybrid fitness. Very strong but incomplete assortative mating can induce low hybrid fitness via a mating disadvantage to rare genotypes, and this can stabilize coexistence of two species at high but incomplete levels of assortative mating. Given these results and evidence that it may take many millions of years of divergence before related species become sympatric, we postulate that coexistence of closely-related species is more often limited by insufficient assortative mating than by insufficient ecological differentiation.

Secondary contact rather than coexistence— Erebia butterflies in the Alps

“…2012). Conversely, even a very small amount of hybridization or just interbreeding itself between lineages with strong assortative mating could suffice to prevent coexistence (Irwin and Schluter 2022). However, if neither ecology, sexual selection, nor their interaction suffices to complete reproductive isolation, competing species may stay in stable parapatry at contact zones (Tobias et al.…”

“…2020; but see M'Gonigle et al. 2012) or when hybridization occurs even under high assortative mating (Irwin and Schluter 2022). As such, secondary contact zones may differ from one another in where they could be placed within the “gray zone” of late‐stage speciation (Roux et al.…”

Initiation of speciation across multiple dimensions in a rock‐restricted, tropical lizard

“…Ecological divergence can mediate premating reproductive isolation by limiting how often individuals of different populations meet. Furthermore, phenotypic divergence resulting from adaptation to different environments (e.g., ecomorphology) can indirectly result in postzygotic isolation by rendering phenotypically intermediate hybrids unfit (Irwin & Schluter, 2022; Schluter, 2009). Ecomorphology is, in general, quite conservative within the H. planiceps complex, with the exception of larger body size in limestone‐restricted lineages; but whether this reflects drift within isolates or habitat‐induced adaptation is unclear and requires further investigation.…”

How does the magnitude of genetic variation affect ecological and reproductive character displacement?

“…However, they also showed that extinction can be avoided by producing a hybrid swarm when hybrid fitness is high. While our model assumes that there are only two outcomes in reproductive character displacement (i.e., extinction or coexistence), we need to carefully consider the three possibilities (i.e., extinction, coexistence or hybridization) for understanding the eco-evolutionary dynamics of speciation by reinforcement/reproductive character displacement (Irwin & Schluter, 2021;Kyogoku & Kokko, 2020).…”