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The effect is much larger than that. For most of the evolutionary history of humans, everyone* lived in Africa. Then only quite small groups left, taking with them only a small fraction of the genetic diversity of humans. Even today, the great majority of genetic diversity among living humans is inside Africa.

(* who mattered. There were earlier migrations of hominids, but the mark they left on our genetics is much smaller than the influence from later migrations.)

I was under the impression humans migrated from Africa so the rest of the world is all african-X, where the african- part is some subset of the original african population.

"One drop rule" makes everyone black in America

More importantly, humans have lived in Africa far longer than anywhere else in the world, so have had much more time to genetically diversify.

It's not just about size.

Humans originated in Africa, so populations there have had more time to evolve and become more diverse.

200,000 years of genetic drift versus 20,000 years makes a big difference.

Also population bottlenecks. Only a handful of populations left Africa, whereas many remained back.