The question is how geographically localized that "well defined group" is. Are Eritreans in the same well defined group as Ghanans or as Lebanese? Are Cypriots the same as Lebanese or Italians? Are Italians in the same well defined group as Greeks or as Germans? Are Germans like Slovaks or like the Sami?

You can surely say that the overlap in a Ghanan and a Sami person's ancestry is quite small regarding recent millennia. But I don't see well defined groups at all. The reason it seems so in the US is because of the huge Western European (German/British) immigration early on and the large population of descendants of West African slaves. These two are sharply distinguishable (discounting for intermixing effects) but there are no "well defined groups" such as "black" and "white" when looking at it globally, not just in America. They are linked continuously via North/East Africans/Arabs/Turks and Mediterraneans.

OK. Data for well defined groups here: https://rosenberglab.stanford.edu/papers/popstruct.pdf

Advice to not interpret any genetic data in a divisive manner here: http://www.ask-force.org/web/Golden-Rice/Yudell-Taking-Race-...

This is an old paper. A lot more work has been done on Japanese and on Kalash (see Reich) and the Altaic language family is on the backfoot these days. Reich himself is also very sloppy with attributions about Dravidian language.

I've studied a wider range of literature, and I suggest to not discuss this question one way or another. Let's leave genetics to geneticists.