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October 27, 2005
More on the Recent Study of Genetic Diversity in Human Populations
The other day I posted on a new report of a study by Sohini Ramachandran that seemed to support the out of Africa hypothesis. At that time, I asked how multi-regionalists would respond and lamented the fact that more details were not available. Well more details are available and guess what; the paper may not support the "out of Africa" hypothesis after all. It more assumes it. Here is the abstract that I couldn't find at the time.
Equilibrium models of isolation by distance predict an increase in genetic differentiation with geographic distance. Here we find a linear relationship between genetic and geographic distance in a worldwide sample of human populations, with major deviations from the fitted line explicable by admixture or extreme isolation. A close relationship is shown to exist between the correlation of geographic distance and genetic differentiation (as measured by FST) and the geographic pattern of heterozygosity across populations. Considering a worldwide set of geographic locations as possible sources of the human expansion, we find that heterozygosities in the globally distributed populations of the data set are best explained by an expansion originating in Africa and that no geographic origin outside of Africa accounts as well for the observed patterns of genetic diversity. Although the relationship between FST and geographic distance has been interpreted in the past as the result of an equilibrium model of drift and dispersal, simulation shows that the geographic pattern of heterozygosities in this data set is consistent with a model of a serial founder effect starting at a single origin. Given this serial-founder scenario, the relationship between genetic and geographic distance allows us to derive bounds for the effects of drift and natural selection on human genetic variation. [emphasis added]
As can be seen in the first highlighted passage, "the data set are best explained by an expansion originating in Africa," the paper does seem to indicate support for out of Africa, but this is only true if one accepts a founder population and that the later populations never interbred. The words "consistent with a model of a serial founder effect," really speak to the limitations of the study. The data may be consistent with other hypotheses also. As John Hawks says,
. . . the paper doesn't prove out-of-Africa. It doesn't even present any new data that support out-of-Africa. It presents some new simulations of how an out-of-Africa dispersal might work, but it doesn't test those simulations by comparing them to data that might differentiate their preferred model ("serial founder effect") from other hypotheses that might explain the same observations. [emphasis in original]
Hawks asked what other hypotheses might explain the data offered by Ramachandran and offers this answer.
One scenario that matches the evidence is multiregional evolution with a recent African dispersal of some adaptive genes.[snip]
A second hypothesis is presented by Templeton (2002), who proposed that several founder effects happened at different times in the Pleistocene, each carrying one or more genetic variants out of Africa.
[snip]
A third hypothesis is that population size was simply greater within Africa than within Eurasia.
Hawk's post on the paper is very instructive. First, he shows, again, how news releases can over state the implications of a study. While, I strongly favor scientists getting their work into the public domain, I believe that great care need be taken not to over state what is being claimed.
Second, he gives us a very useful discussion on alternative explanations of the same data. While I tend to think the out of Africa hypothesis makes the most sense of the available data, it is far too early to be dogmatic on this point. Hawk's shows us why.
Posted by Duane Smith at October 27, 2005 7:34 PM | Read more on Paleoanthropology |
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Comments
The second edition of Conroy's Reconstructing Human Origins has a good discussion of the issues Hawks raises - including the Templeton paper.
Posted by: afarensis at October 28, 2005 8:43 PM
I've consulted Conroy a few times in the library but I guess it's time to get my own copy. Right now, my book budget is a little strained by trying to get my Semitics studies library up to date. 35 years is a big gap.
I'd really be happy if academic publishers would start to use print on demand technologies so it would be possible to buy this kind of book in the $25.00 range rather than the $75.00 range. I have this complaint concerning biology, anthropology, archeology and Semitics technical books. Only a very few publishers use modern methods to get costs down. The Abhandlunger zur Literatur Alt-Syrien-Palästinas series from Ugarit-Verlag is good example of how to do it, if only I could get them to publish all their books in English.
Sorry for the irrelevant rant about book prices.
Posted by: Duane at October 28, 2005 9:37 PM
I bought mine used so it was about half that.
Posted by: afarensis at October 31, 2005 4:11 PM
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