There are relatively few genomic regions that differ substantially among populations. Yet, based on continental origin and ethnogeographic affiliation, some phenotypes (e.g., skin color, height, facial features, and hair textures) exhibit substantial variation as a function, seemingly, of genetic ancestry. Given the substantial interindividual variability in admixture proportions within most historically intermixed populations, the relationship between overt phenotypes and genetic ancestry (or social constructs) is tenuous. For example, dark skin color imparted by eumelanin expression would not be a good indicator of West African ancestry, since many other populations such as Australian, Melanesian, and South Asians also express higher levels of eumelanin and exhibit darker skin color. In other cases, cryptic population structure contributed by recent ancestral admixture can be common for many populations, yet not always appreciable and certainly not quantifiable through self-assessment or visual cues. Hence, the practice of binning persons into single population groups can be inaccurate, and can confound genetic associations contributing to both type I and II errors.
Naturally, if Mark Shriver wrote it, it must be true. However, I'm unclear why the heavily-couched and citation-free assertion warrants my attention. The claim concerns "historically intermixed populations"--e.g. Aframs and mestizos--rather than white Americans.
The population structure of white Americans has been investigated at resolutions much higher any ever used by DNAprint. What (not so) cryptic population structure did researchers uncover? You should already know this:
European Americans are often treated as a homogeneous group, but in fact form a structured population due to historical immigration of diverse source populations. . . . components roughly corresponding to northwest European, southeast European, and Ashkenazi Jewish ancestry are the main sources of European American population structure.
[Discerning the ancestry of European Americans in genetic association studies..]
Likewise for Europeans:
European population genetic substructure was examined in a diverse set of >1,000 individuals of European descent, each genotyped with >300 K SNPs. Both STRUCTURE and principal component analyses (PCA) showed the largest division/principal component (PC) differentiated northern from southern European ancestry. A second PC further separated Italian, Spanish, and Greek individuals from those of Ashkenazi Jewish ancestry as well as distinguishing among northern European populations.
[Analysis and application of European genetic substructure using 300 K SNP information.]
Put it to sleep. White Americans are not "historically intermixed" with nonwhites to any significant degree, however much you might wish it to be so. Nor is there any evidence "ancient admixture" is an important source of genetic structure in Europe (at least within the major population blocs). The relevant distinction is not between those who have a Nigerian or Mongol in the woodpile and those who don't, but between those of Northern or Central European origin and those of Southern European origin. You know which you are. Your 100% IE certificate can't change that.
(*) I am the "phenotypists" (sic) in question. I recognize the existence of no such category. It is practically a tautology to note that the definitive answers to questions of genetic relatedness are to be found in the genome. But not quite yet and not by Rienzi's favorite company. ABD tests 176 SNPs. Out of about 10 million. (Then there's the other 2.99 billion base pairs.) For now, I'll trust my eyes, thanks. This man is not 21% East Asian, and telling him he is does not advance European interests--no, it led a white man to start speculating about descents from Attila.