I have no problem with a genetic explanation of the probabilities in our sport (not that I always agree, but a genetic explanation like the one you gave is at least a good place to start the debate). But I just wanted to call B.S. on the 'not a fast twitch people' remark.
Plus, it seems like people are defining 'fast twitch' in all different sorts of ways. It looks to me as though MLS players (also note here folks like Ichiro and Matsui, but could be anybody) is a 'fast twitch' athlete, right? If stealing a base or getting a home run or an ippon in judo is NOT fast twitch, then I don't know what fast twitch is.
I think that Jason's comment still stands, that the Japanese have huge depth at the marathon, although no male superstars for quite awhile, for largely cultural reasons. Japanese are good at Judo for largely cultural reasons. Americans used to suck and are now getting much better at soccer/football for cultural reasons.
When it comes to distance running or sprinting you want people with light builds who lead active life styles. You'll find a lot of people who fit that bill in the East African country-side; a mix of genetics and culture. No doubt a similar mix of explanations accounts for sprinting domination among those with West African genes.