Interestingly, the current mating and segregation trends, to the extent they exist, appear to be completely "natural" and lacking in premeditation. Can't help but wonder if modern humans are not the result of a process that left the Neanderthals ( pre-modern humans, whoever they were) behind. I'm no expert, but I don't think a complete lack of gene flow between groups is thought to be required for the process of speciation.