I don't necessarily agree -- or disagree -- with the article posted below, but I thought I'd throw it out there:
http://www.pponline.co.uk/encyc/0657.htm
Of course, the article divides groups up on more than just race, so I'm not sure it's exactly what we're arguing here. And it touches on running more than other sports, but I think what it says can be applied to basketball and, to a lesser extent, baseball and football.
I have to admit, however, that as a runner I hear tons about the Kenyans dominating the distance sport and there has got to be more of a reason for that than just training -- although race isn't necessarily it. But the "more slow-twitch muscle fibers" argument makes sense to me, if it's true.