Fundamentally, the problem is of what constitutes a race. Similar skin tones, or living on the same continent, are pretty useless determiners since they don't tell us anything.
As for dogs, which are much simpler creatures in the first place, they have in terms of breeds been systematically bred to exaggerate specific attributes.
This has never been done with humans. Now, if we were to spend a thousand or two years cross-breeding the most successful humans, bolstering this with genetic engineering, maximal quality of diet, and so on, we could expect to produce a legitimate "race".
Until that happens, get a grip - there are no races or breeds of men that come even close to matching up with preconceived sociocultural categories. "Black", or "sub-Saharan African", is simply not a productive racial classification if you're looking for genetic regularities.
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