Coexistence in sympatry with gene flow before speciation has completed

We also used genomic simulation with assortative mating in an ecological model to determine how much selection is required to reproduce observed genotype frequencies. Results from the two approaches were comparable and yielded estimated selection coefficients S against the least fit ancestry proportion (within backcross range) between 0.5 – 0.6. Surprisingly, selection was found to be only slightly weaker than that leading readily to collapse and fusion in our simulations, suggesting that sympatric stickleback species are close to a coexistence boundary.

Moderately strong selection appears to be required for coexistence with even low levels of gene flow. We suggest that larger niche differences are required to stabilize coexistence with gene flow than without gene flow. This helps to explain why successful sympatric species that hybridize often show conspicuous ecological and phenotypic differences.