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Thursday, February 28, 2008

More on "something to ponder"

This post is a continuation of Something to ponder, posted a few days ago.

In PLOS-Genetics, March 2007, C. M. Nievergelt et al. published an article with the quite unattractive name of General Analysis of Molecular Variance. In the article they published this neighbour-joining tree:

(For the legend and better image access it directly: HERE).

When compared with the more recent graph (of Jun Z. Lin et al) commented before, the similitued is striking: West Eurasians are again closest to Sud-Saharan Africans, while East Asians are the most distant ones. Sadly there is no data in this graph re. South Asia but another thing is noticeable: unlike in Lin's result, here Amerindians (or rather the root of the American branch) are closer to the root than Oceanians (New Guineans only in this graph). But as the American branches are also much longer, this may partly explain that difference.

Leaving aside this divergence, the two graphs are surprisingly similar in placing East Asians as the most divergent group from the tree root (independently on wether you place it inside the African branch or at the African-Eurasian divergence point). The authors mention that the distance from Addis Ababa is the strongest predictor of genetic background similarity among the individuals and populations. But China is still much closer to Addis Ababa than North America unless you consider the transatlantic route (which we know was not the case).

Nievergelt et al. add: but the world regions explain variation in genetic background similarity over and above this measure, suggesting that diversity among individuals within populations situated within the same world region is not completely captured by their distance from Addis Ababa. Worthy note indeed but still unable to justify the extra genetic distance of East Asians.

I am still pondering the why of this anomaly.

In other notes, in this graph it's quite noticeable the apparent homogeneity of European samples and their belonging to the larger West Eurasian group. Inside this, Near Easterners show much larger differences among them than Europeans do. Another clear single branch (for those that still speculate with several prehistoric migrations into America) is that of Native Americans.


Manjunat said...

I am still pondering the why of this anomaly.

I think there was a recent paper that observed that East Asians underwent population bottlenecks. I don't remember it clearly.

From bottleneck to population explosion might some way cause for this?

Maju said...

That was precisely what I mentioned in the first post. At least it's the more reasonable explanation I can come with.

Still that research suggested that East Asians had gone through only slightly (yet maybe significative) narrower or longer bottlenecks than Western Eurasians. Maybe that's all but what about American Natives? They also experience a marked bottleneck and long coalescence in Beringia it seems.

I still can't get a final answer. In any case the extra distance may not be that big anyhow and I'm just emphasizing a minor detail. ??