That appears to be the case. We don't know for certain due to lack of controls. I have no problem with contending for selection since signs point in that direction.

It actually makes very little sense that African Americans would not be more suited for labor than Africans since they indeed were selected for genetically. Just a couple generations of selective breeding can drastically change the genetic landscape.

I believe Yao Ming is the product of only one generation of selective breeding (two tallest athletes in China).