The race deniers still seem to ignore what they want to ignore.

"little evidence of further substructure within the other racial populations"

They seem to be implying that there has been interbreeding between different ethnic groups within the larger racial groups, something that casual observation seems to suggest. It's not uncommon for American whites to say things like "my ancestors are Irish, Polish, German, etc., etc.," and these European groups were supposedly somewhat interbred to begin with. The same thing may be true for blacks because although they came from a relatively large area of a West African coastal region they adopted a generalized "black" identity because after some time, the memory of specific ethnic groups within Africa was lost. There apparently is a trend for black with a large degree of white admixture to marry other part-white blacks, but it sounds like that wouldn't show up in a study like this because although the lighter blacks may think of themselves as different, they still just call themselves "black".

Thanks for pointing out these two papers. I'd always been under the impression that self-reported ethnicity was unreliable in certain highly admixed populations. Looks like I was both right and wrong. It will be interesting to see how admixture will change the genetic profile of populations over time in multi-ethnic countries like the U.S.