September 02, 2009

Y chromosomes and mtDNA of Central Asian Turkic and Iranian populations

Unfortunately this paper only studied 12 Y-STRs, reduced to 7 to compare them with previous studies. Moreover, as far as I can see, this data is not available in the journal website. ScienceDaily covers the paper with the totally unwarranted title of "No Such Thing As Ethnic Groups, Genetically Speaking, Researchers Say".

What this paper does show, as far as its limited marker set can, that some Turkic ethnic groups are aggregates of populations of unrelated origin, which is not particularly surprising. One has to look at the spread of Turks from Central Asia to Europe to see that the various "Turks" were usually opportunistic alliances of peoples of different stock. Perhaps these Central Asian ethnic groups will eventually be homogenized by continued in-group marriage.

This brings me to an important point of using genetic diversity to assess how long ago ethnic groups were formed. Some ethnic groups begin as homogeneous entities which become differentiated as they expand and undergo separate evolution/differential patterns of admixture in different localities. Other groups begin as heterogeneous groups of unrelated tribes that become united by some factor, e.g., the emergence of a strong king or dynasty around whom diverse peoples aggregate. In the first case, ethnic evolution is one of diversification over time, as the genetic legacy of the homogeneous founders is fragmented; in the latter, it is one of homogenization, as the genetic legacies of the heterogeneous constituents merge to form a single homogeneous group. Thus, one can't generally conclude, by looking at within-group differentiation whether the group is "old" or "recent" in origin; it could be an old group that has fragmented over a long period of time, or a new one that has not had enough time to become one.

UPDATE (Sep 3):

The study is also covered at the Spittoon under the title New Study on Genetics of Ethnic Groups Reveals We May Not Be So Different After All. Unfortunately, the author of the blog post gets the linguistic divisions wrong:
The Turks are largely nomadic herders. They speak Indo-Iranian languages like Azerbaijani, Turkish, and Altay. Their society is organized into clans, or “descent groups,” whose membership is passed down from father to children.

The Tajiks are, conversely, agriculturalists. They speak various dialects of the the Tajik, or Tajik Persian, language that may have arrived with Muslim invaders 1,000 years ago. Their society is largely patrilocal – meaning that when couples marry they put up residence near the husband’s family; and first cousin marriages are encouraged.
It is of course the Turks who speak Turkic languages, and the Tajiks who speak an Indo-Iranian (or more precisely Iranian) language.

As for the title, which, like the ScienceDaily title, seems to burst at the seams with delight that ethnic differences don't exist, a better angle on the topic would be to observe how prevalent ethnic differences are, if they can exist even among populations that are genetically non-differentiated. The pipe dream of some thinkers, that increased inter-ethnic and inter-racial mating will lead to an abolition of ethnic and racial genetic differences, and, thus, to a happy co-existence of people around the world, is refuted by the finding that humans happily self-segregate themselves along ethnic lines, even when there are no underlying genetic differences.

BMC Genetics 2009, 10:49doi:10.1186/1471-2156-10-49

Genetic diversity and the emergence of ethnic groups in Central Asia

Evelyne Heyer et al.



In this study, we used genetic data that we collected in Central Asia, in addition to data from the literature, to understand better the origins of Central Asian groups at a fine-grained scale, and to assess how ethnicity influences the shaping of genetic differences in the human species. We assess the levels of genetic differentiation between ethnic groups on one hand and between populations of the same ethnic group on the other hand with mitochondrial and Ychromosomal data from several populations per ethnic group from the two major linguistic groups in Central Asia.


Our results show that there are more differences between populations of the same ethnic group than between ethnic groups for the Y chromosome, whereas the opposite is observed for mtDNA in the Turkic group. This is not the case for Tajik populations belonging to the Indo-Iranian group where the mtDNA like the Y-chomosomal differentiation is also significant between populations within this ethnic group. Further, the Y-chromosomal analysis of genetic differentiation between populations belonging to the same ethnic group gives some estimation of the minimal age of these ethnic groups. This value is significantly higher than what is known from historical records for two of the groups and lends support to Barth's hypothesis by indicating that ethnicity, at least for these two groups, should be seen as a constructed social system maintaining genetic boundaries with other ethnic groups, rather than the outcome of common genetic ancestry


Our analysis of uniparental markers highlights in Central Asia the differences between Turkic and Indo-Iranian populations in their sex-specific differentiation and shows good congruence with anthropological data.



Unknown said...

in the latter, it is one of homogenization, as the genetic legacies of the heterogeneous constituents merge to form a single homogeneous group.

So how does the homogenization happen - some alleles selected against and bred out? Does this always happen? (I don't think so, but may be wrong) How long - order of magnitude - does the homogenization process take? Examples of homogenized populations?

Dienekes said...

The Uyghurs are an example of a homogenized population that was formed in the order of a single millennium. Originally composed of Caucasoid and Mongoloid elements, by now it is a stable (almost 50-50) of the two, in which most individuals show similar ancestral proportions. This contrasts with other admixed populations (e.g., Mestizos or African Americas) where widely different ancestral proportions are observed, probably because (i) not enough time has transpired, and (ii) admixture has been ongoing (e.g., Barack Obama) until the present.

Dienekes said...

Another great example is that of Iceland which demonstrates both processes admirably. The "Norse" and "Scottish" elements have been largely homogenized (there are no sub-populations of much higher "Norse" or "Scottish" ancestry than the national average), but new regional differences (not related to ancestral proportions) have emerged in the country.