If they do hybridize with them the resulting offspring are sickly, or sterile like mules. So natural selection penalizes any predilection, on the part of individuals on either side, towards hybridizing with the other species or even race. Natural selection thereby finishes off the process of ‘reproductive isolation’ that began with the chance intervention of a mountain range. ‘Speciation’ is complete. We now have two species where previously there was one, and the two species can coexist in the same area without interbreeding with one another.
