Extinction and introgression in a community of partially cross-fertile plant species.

We develop a model to study the demography and genetics of an encounter between two partially cross-fertile plant species. We assume prezygotic reproductive isolation between the species, a common situation when the species differ by their phenology or floral traits that cause assortative mating. Three outcomes are possible: coexistence of both species with minimal introgression; domination by one species, with the other becoming extinct or surviving only through recurrent migration; or domination of the community by hybrid derivatives, with both species surviving but with a rather high level of introgression between them. The first situation is reached when interfertility is low, while the third requires high interfertility to develop. Occurrence of the second situation is observed with intermediate values of interfertility. Gene flow from nearby monospecific populations can prevent both introgression and the domination of the community by one species. Conversely, increasing the number of loci that determine the reproductive isolation between species or decreasing the degree of nonadditive interactions (epistasis and/or dominance) between alleles and loci makes introgression more likely. We found that hybridization can create positive frequency dependence and make extinction possible, even when hybrid individuals have no intrinsic fitness advantage.