The Nazis were always terrified of the mongrelization of the human race, seeking to breed a race of purebred ubermensch, as if the human family was a farm or a racing paddock. The truth is mongrelization is good, that keeping the gene pool mixed up is the best way to ensure the species is prepared for whatever changing conditions on the planet will throw at it, and further, that this is what nature does anyway, if left alone and given the chance. Highly specialized creatures and homogeneous species only arise when conditions are unnaturally stable for too long, and populations do not move for long periods of time, and they suffer greatly when things change, as they will inevitably. Purebreds are inbred cripples incapable of adapting.

Natural selection, unlike human-directed selection, breeds not for the most perfect organism for one circumstance, but for the most diverse one possible for all circumstances. It is only through diversity that we can specialize in a hurry if we need to, and to generalize again when we inevitably have to. In nature, separate isolated human populations arise to meet local conditions. But this is only in response to environmental stress. It is only possible because we possessed the diversity to make it possible to evolve in any direction we needed to.

Furthermore, I suspect this basic principle holds true in any evolving analogue of biology, say, in cultural evolution. Mongrelization works, we need it in order to be adaptable.