It is considered acceptable to discuss the differences between the "breeds" of the various non-human animals (ie horses, dogs), but we certainly get into socially dicey territory when we discuss the various human "breeds".
I have pondered if the harsh environments involved, in the history of African-American slave trade and use, might have had the potential of a semi-natural selection process leading to a geneticly stronger population. This line of thought tends to focus toward concepts of physical superiority, which can then be erroniously tied to a concept of intellectual inferiority.
But I'm starting to sound like Brad.
A very socially tense topic to explore - Probibly best stayed clear of.
Moose