NOTE: I inadvertetdly posted a draft of this post. Here is the final post; I will leave the draft online since some people already commented on it, before I noticed it.
A previous study on Europeans discovered that Caucasoid Europeans, who form a genetic cluster on a global scale can be further distinguished into subclusters that are correlated with ancestry and geography. Now, a new study on European Americans (hat tip gnxp) appearing in the free online journal PLoS Genetics has carried out a similar analysis of the genetic structure of American "Caucasians".
What I find fascinating about this new study is that an ethnic subgroup within American Caucasoids, namely Ashkenazi Jewish Americans can be distinguished at this point from other Caucasoids. Here is the clustering based on the validated set of markers from the paper:
The distinctiveness of Jewish Americans is probably due to their having a portion of Middle Eastern ancestry. We can only say that Jewish Americans are clearly genetically distinct from the other ethnic groups presented in the study, although it is unclear whether they are distinct from other groups of Middle Eastern background.
As biologically-averse intellectuals continue to question the very existence of race, pragmatic scientists are moving into an era when not only race, but ethnicity may become genetically identifiable.
Unlike race which by definition refers to an identifiable biological cluster, ethnicity may (or may not) refer to such a cluster.
Ethnic distinctiveness is due to both culture and biology, and the relative proportions of the two factors are specific to a particular ethnic group.
When ethnic groups have split recently, co-inhabit a geographical space, frequently intermarry, etc., then it is likely that they will have small biological differences, whereas other ethnic groups may be biologically as well as culturally distinct.
We should be careful to note that there are two levels of ethnic biological distinctiveness: group distinctiveness and individual distinctiveness:
Group distinctiveness means: if you are given the photographs of ten Englishmen on one side and ten Russians on the other, you would be able to decide with a very high level of success which group represented the Russians and which one the Englishmen.
Individual distinctiveness means: if you are given the photographs of ten Englishmen and ten Russians in random order, you would still be able to sort out the Russians from the Englishmen; whether this is possible, and with what level of success is less obvious than in the previous case.
We must wait for more studies with larger samples and more markers to study the biological component of human ethnicity. At present, some groups do seem to have individual distinctiveness in a particular societal context and with a particular set of markers (e.g., Jewish vs. non-Jewish Americans), while others are less distinct (e.g., Greek vs. Italian Americans).