It's interesting that running fast is seemingly the end all be all of "athleticism". Why are most of the fastest cyclists
or swimmers white or the strongest power lifters? Where do those and other feats rank in athleticism? In my opinion a lot of the "best" boils down to how much time is spent on something culturally. Small wonder Jamaica produces fast athletes they embrace the sport. Iceland embraces power lifters so small wonder you see a predominate number of strong men.
Gentics is the seed but IMO culture is the nutrients that help feed said seed.
Kenyan runners are another fantastic example where gentics may play a role but not the only role. For starters it's not all of Africa or even Kenya that produces great long distance runners. Instead it is a specific minority tribe. They gentically have thinner ankles which is one key. Take that further though and they additionally embrace pain and the sport itself. Pain tolerance being of importance in long distance running.
How One Kenyan Tribe Produces The World's Best Runners : Parallels : NPR