First dispersal and then vicariance were involved in establishing the current distribution of the family Tapiridae. Use the fancy words that belong in this discussion. What form of natural selection was probably necessary for the speciation at Chichancanab, but not required for the speciation at Death Valley. At 250 mybp there was a sudden drop in brachiopod species and an adaptive radiation of bivalve mollusks followed. Ever since, the bivalve mollusks have increased in species diversity and the brachiopods have held their own, but with only a few extant species.
Some species have exceptionally strong and specific instincts for habitat selection.
But that is not the one I am interested in here of course. Dispersal by diffusion is the more interesting point to be made here. The spread into Eurasia would have had to occur through Beringia or across the Atlantic land bridge, one of which must have been exposed and had a tropical or subtropical climate some time in the history of the lineage. D. Vicariance got involved when the modern relictual populations were separated, both by extinction of intervening populations, probably because of climate change and by submergence of the land bridge used to colonize Eurasia. The Lago Chichancanab pupfish had to speciate sympatrically because they did so in a single lake with no geographic barriers.