Keep in mind the culture of the various regions could easily cause fast natural (?) selection. I’m no expert on Japan’s history, and even less so on China/Korea/etc. I know enough that Japan had periods of pretty constant war, and that the young and the brash would essentially volunteer to be mercenaries. It seems to me that could pretty quickly weed out the most aggressive genes in a population. It would be different, of course, if soldiers were conscripted instead.
That’s just one example and as I said, I’m not a historian. Another example would be America. People always come up with silly reasons why we became the dominant world power. In my opinion the reason is simple: the people that immigrated here were risk takers, hard workers, and dreamers. Their kids would inherit those genes.
So, I’d say culture begets genetics which begets culture…