Introgressive hybridization in response to environmental change: The role of assortative mating

Background/Question/Methods   Rapid evolution on ecological time scales can play a key role in species responses to environmental change such as climate change. In addition to being critical to the response of species with constrained capacities for range shifts and/or acclimatization, rapid evolution is likely to be particularly important in cases with sudden environmental shifts, as might occur with climate-change-driven changes in the magnitude and frequency of extreme events. One dynamic that has the potential to generate the diversity necessary for evolution rapid enough to allow response to environmental shifts is introgressive hybridization, which has played a key role in adaptation to novel environments in a diverse array of taxa. However, if distinct sub-species exist before an environmental shift, mechanisms that impede hybridization, such as assortative mating and hybrid inferiority, are likely to be present. Here I explore the theoretical potential for introgressive hybridization to play a role in response to environmental change. In particular, I incorporate assortative mating, hybrid inferiority, and demographic stochasticity into a two-locus, two-allele population genetic model of two interacting sub-species where one locus identifies the species and the other determines how fitness depends on the changing environment.

Results/Conclusions   Simulation results indicate that both having separate sub-species before an environmental shift and have introgression after the shift depends on a balance of assortative mating strength, hybrid inferiority, and the change in fitness in the focal allele. In particular, moderately high values for the strength of assortative mating will allow enough hybridization events to outweigh demographic stochasticity but not so many that continued hybridization outweighs backcrossing and introgression. High values for recombination between the two loci further enhance the potential for introgression. Successful introgressive hybridization also requires intermediate relative fitness at the allele negatively affected by environmental change: this balance allows high enough fitness such that hybrid survivorship outweighs demographic stochasticity but not so high that the change in selection is too weak to affect the genetic dynamics. The potential for extinction rather than successful hybridization at lower fitness is larger with promiscuous rather than monogamous mating due to greater stochasticity in mating events. Overall, these results indicate species characteristics (e.g., intermediate assortative mating and mating systems with low variation in mating likelihood) which indicate a potential for rapid evolution in response to environmental change via introgressive hybridization.