Hybridization and the coexistence of species: HZAM-Sym code and data files

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. We model coexistence outcomes for two sympatric
species that are ecologically differentiated but have incomplete
reproductive isolation. 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. High intrinsic growth rates result in higher reproductive
rates when populations are below carrying capacity, reducing the
probability of extinction and increasing the probability 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.