Two species of barkflies (Order Psocoptera) allopatrically speciated from each other 200,000 years ago when their island habitat split into two islands with rising sea levels. The barkflies on Island 1 specialized in feeding on a toxic lichen species, while barkflies on Island 2 fed on fibrous algae that were difficult to process and digest. Once sea levels fell, the islands once again became one continuous land mass, and the barkfly species began to intermingle. Hybrids between the species were generally incapable of eating either the toxic lichen or the fibrous algae. This meant that hybrid individuals were heavily selected against, and each parent species began to evolve adaptations to the genitalia to avoid breeding with the opposing species. Thus, the two species remained genetically isolated despite re-contact. This is an example of: