Carl Zimmer pointed me to a new paper, A genome-wide genetic signature of Jewish ancestry perfectly separates individuals with and without full Jewish ancestry in a large random sample of European Americans. The title is so informative that pasting the abstract is almost unnecessary, but here is the conclusion which gets to the point:
In conclusion, we show that, at least in the context of the studied sample, it is possible to predict full Ashkenazi Jewish ancestry with 100% sensitivity and 100% specificity, although it should be noted that the exact dividing line between a Jewish and non-Jewish cluster will vary across sample sets which in practice would reduce the accuracy of the prediction. While the full historical demographic explanations for this distinction remain to be resolved, it is clear that the genomes of individuals with full Ashkenazi Jewish ancestry carry an unambiguous signature of their Jewish heritage, and this seems more likely to be due to their specific Middle Eastern ancestry than to inbreeding.
There have been other papers which show that Ashkenazi Jews form a separate cluster from gentile whites in the United States. This is important again in the context of biomedical studies attempting to ascertain the genetic roots of particular diseases; population substructure (e.g., Jew vs. non-Jew) may result in confounded associations. Also, one of the authors of the paper is David Goldstein, author of the fascinating Jacob’s Legacy.
In any case, on to the PC charts where the real action is. Do note that I’ve resized and added explanatory labels here & there for clarity.
As you can see, there is almost perfect separation between the Jewish and non-Jewish clusters here. PC 1 = first principle component of variation, and PC 2 second principle component of variation. These are the two largest independent dimensions of genetic variance extractable out of the data set. The authors note that there is almost perfect separation along PC 1, and, they note that most of the gentile whites who are closest to the Jews on this PC are of Italian or Eastern Mediterranean origin. This is important later.
As you can see here, the interesting point is that Jewish ancestral quanta is roughly predictive of genetic position. This shouldn’t be that surprising once we know that Jews and non-Jews separate so cleanly (e.g., someone who is biracial would be located between their two parent racial clusters on any plot), but it is striking nonetheless in reaffirming the genetic reality of Jewishness. The United States has an easy genealogical history for Jews, as the ancestors of all self-identified American Jews today arrived on the order of 4-6 generations ago. These individuals were likely Jewish relatively far back in their lineages, and admixture is easy to recall because it is of recent vintage.
Finally, let’s put Ashkenazi Jews in the worldwide context.
I labeled a few populations for clarity. Orcadians are individuals from the Orkney Islands, just off the northern coast of Scotland. As you can see, American whites occupy an expansive region, in large part due to their diverse origins and admixtures. The Druze have been genetically isolated for over 5 centuries, so it is not particularly surprising they form a distinct cluster. It is likely a function of drift due to bottlenecking, just like Iceland. The separation between the Ashkenazi and the Palestinians might be of interest to some because of political reasons.
As the authors note, the Ashkenazi are almost certainly a compound population. There are full-Jews who “look gentile” and full-Jews who could pass easily in the Middle East as a native. Most Ashkenazi Jews exhibit a mix of features. The genetic likelihood that the Ashkenazi have varied origins should no surprise. The paper notes that Askhenazi heterozygosity is actually somewhat greater than American whites, implying that the source of their distinctiveness has to do with genetic origin, not genetic history (i.e., population bottlenecks). Another population which I suspect resembles the population history of the Ashkenazi are Uyghurs, who fall between Europeans and Han Chinese in their ancestry. But the hybridization even occurred in the past, so the Uyghurs today can be thought of as a separate population with its own suite of genetic combinations, as opposed to a hybrid of two “pure” groups. Similarly, Ashkenazi Jews no doubt emerged from hybridization evens between Middle Eastern and European females, but over the generations they have been relatively endogamous (most gene flow has been out of the community into gentile Europeans through conversion) and so are a genetically coherent population in their own right.
Note that the authors had only a few non-Ashkenazi Jews in their sample, so the assertions above only apply to the Ashkenazi (95-99% of America’s Jewish population, but only ~50% of Israel’s).
Update: Dienekes emphasizes that the hypothesis that the admixture element which makes Ashkenazi Jews distinctive from other Europeans is Middle Eastern is somewhat speculative. I’m less skeptical than him seeing as how some Y lineages are Middle Eastern, but he offers this interesting point:
With that said, I do suspect that the distinctiveness of the Ashkenazi Jews is in part due to the possession of a Middle Eastern component of unspecified strength. I base this hypothesis on the results reported to me about the EURO-DNA-CALC test. This test distinguishes between NW, SE Europeans and Ashkenazi Jews; a few Arab individuals who have communicated their results to me have reported fairly high AJ components, indicating that part of what distinguishes an AJ from Europeans is related to the Middle Eastern Semitic background of that group.