I suppose if some group of people got themselves into their own species, where they could not in fact mate with what we know as "human", it would be a pretty big deal.
Edit-- I quoted the same line... please feel free to respond to my comment below.
Probably the most obvious example is http://en.wikipedia.org/wiki/Sickle-cell_disease#Epidemiolog... which is a condition that mostly appears in people with ancestry in regions that expose them to malaria.
Skin color is obviously a biological difference that distinguishes different human populations, it just doesn't correlate with all the stuff we actually care about, for instance intelligence. 'Race' is loosely correlated with some specific biological traits, for instance http://www.webmd.com/hypertension-high-blood-pressure/guide/...
As for hyper-successful people breeding themselves into a sub-group with specific traits, for better or for worse, this is obviously possible http://en.wikipedia.org/wiki/Prognathism#History