The second main cause of geographically distributed genetic variation is due to non-uniform sampling of a population. The main cause is founder effect, this is the effect of a small group of individuals migrating from a larger group and founding a new population, if the migrating population represents only a small subset of the parental population, then it will not be genetically representative of the parental population (sampling error). Small founding populations are also subject to genetic drift, which may further alter allele frequencies. An example of this is the human migration out of Africa, it has been theorised that the migration out of Africa only represented a small fraction of the genetic variation in East Africa, and that this is the cause of the observed lower levels of diversity in all indigenous non-African humans.
http://en.wikipedia.org/wiki/Human_genetic_variation
The only way an American will ever compete with the Africans is to either a: hope that a rare genetic talent tries out and trains properly, or b: institute a national long distance talent search.
Whites are a generic adapted race. Africans are a representation of the physical constraints of humanity. Running just happens to be one of the sports that anyone can try out.