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.

Methods
This Dryad package contains data files and Julia and R code to run HZAM-Sym (Hybrid Zone with Assortative Mating, Sympatric case) simulations and to generate data figures shown in this paper:

Irwin, D., and D. Schluter. Hybridization and the coexistence of species. bioRxiv. https://doi.org/10.1101/2021.04.04.438369

Note that the first bioRxiv version of the above manuscript was based on only the R code, whereas the second version incorporates results of the same model written in and graphed using Julia.

Please see that second version of the bioRxiv paper for a full description of the model that produced the data.