Monday, April 14, 2008

Europeans, Jews and Middle Easterners   posted by Razib @ 4/14/2008 10:26:00 PM

Greg's post about SNPs, Jews and evolutionary genetic parameters has been getting a lot of play around the blogs & forums. Most of it seems to be due to the persistent interest in the genetic relationship of Ashkenazi Jews to other European populations. This makes sense, since the 19th century the question of how the "Jewish race" relates to European gentiles has had some sociopolitical relevance.... But a commenter at Steve's blog pointed out that Bauchet et al. from last year had a PC chart which included Armenians, who are I think a good proxy for northern Middle Eastern populations in general. One interesting result from surveys of Y chromosomal lineages is the finding that Jews may have more affinities with northern Levantine & Anatolian Middle Eastern populations than with southern Levantine and Arabian ones. The non-trivial female mediated input of Sub-Saharan ancestry into many Arab populations since the rise of Islam is far less evident in non-Arab Muslim populations (Kurds, Persians and Turks) as well as Middle Eastern Jews, and obviously Ashkenazi ones. But another point is that recent work suggests that the impact of historical events (e.g., the Arab conquest) might have been more demographically significant than we had previously assumed, and so Jewish affinity with northern Middle Eastern populations may reflect that these groups have been less affected by exogenous genetic inputs within the last 2,000 years.

Caution about the sample sizes of course (though I assume within the next year we'll have much better data to go off of), but something to include into your list of priors when making phylogenetic background assumptions.

Note: I added geographic labels to the PC chart for clarification.

Update: Steve has another post up:

On the first two axes, Ashkenazi Jews are rather close to "Europeans" and "Russians." They are similar to Yemenites (from Southern Arabian peninsula) on the first axis, but not on the second. And they are similar to Samaritans (who currently subsist on two hilltops in Israel), good, bad or indifferent, on the second axis but not on the first. They are fairly similar to the Druze (of Lebanon and Israel) on the first two axes, but not on the third.


So, Ashkenazis look pretty European on this chart compared to a few Middle Eastern groups. But, as the recent graph showed, genetics has progressed to the point where Ashkenazis (at least those with four Ashkenazi grandparents) can now be reliably distinguished from other Europeans.

The Samaritans are cousins of the Jews. But:
In the past, the Samaritans are believed to have numbered several hundred thousand, but persecution and assimilation have reduced their numbers drastically. In 1919, an illustrated National Geographic report on the community stated that their numbers were less than 150.

Like the Kalash or Sardinians the Samaritans are going to be weird outliers because of their demographic history. Inbreeding and no gene flow in for that long will do that to you (many people in the Middle East are descended from Samaritans of course, but very few Samaritans are descended from non-Samaritans).

The Yemenites are also a peculiar comparison point because they are geographic outliers in relation to other Middle Eastern populations with a long and distinct history. They have a large proportion of Sub-Saharan ancestry for an Arab group. An interesting historical note is that during the Islamic expansion Yemenite tribes were prominent in Iraq and Egypt, though I doubt they left a very strong genetic imprint in these regions.

The Druze are a better point of comparison, being a more mainstream Middle Eastern group. But that's only relative to the Samaritans, who are at an advanced stage of pedigree collapse, or the Yemenites, who are on the geographic margins of the Near East (it is easy to argue that before Islam Yemen was more a part of the trans-Indian Ocean world than it was of the Near East). The Druze are an esoteric ethno-religious group which as been resident in the mountains of Lebanon. who have not accepted converts since 1031, so again you have a recipe for some genetic distinctiveness developing because of social norms.

All that being said...perhaps as we explore the genetics of the Middle East further we'll find that most groups exhibit these sorts of inbred tendencies because of the prevalence of consanguinity?

Addendum: Modest levels of gene flow are very good at equilibrating and mitigating the build up of variation between groups. Islands, like Sardinia, often develop unique genetic profiles because water seems to be a powerful barrier to marriage connections. The Samaritans & Kalash have not had any gene flow in for a very long time, in both cases in part because of being embedded among Muslims who do not generally tolerate conversion to other religions, and in the case of the Kalash their geographic isolation. Some of the same issues apply to the Druze, though I suspect much more modestly (in part because Druze isolation is more recent).