Your sports analogy completely blows up when you look at a sport that controls for both heredity and environment -- soccer. In that sport, the world's top athletes come from a wide variety of ethnic, socio-economic and geographic backgrounds. The world's current best player, Kaka, is an upper-middle class white Brazilian with obvious European genes.

The "black running back" phenomenon is almost entirely explained by environment -- blacks, particularly in football hotbeds of the south, have far fewer economic opportunities and are often steered into playing the position because it's the easiest way to get attention from college scouts and earn a scholarship (RBs at the high school level handle the bar far more and are more important to a team than the QB).