December 22, 2005

More revelations about the genetics of race

Everybody in the blogosphere is having their say about the Dover judicial decision against teaching Intelligent Design, but 95% of those crowing over Dover would be horrified if any public school tried to teach the latest findings about about the evolution of human biodiversity. Last night, we saw the release of Moyzis and Wang's epochal paper with a list of 1,800 different genes that have been under different selective pressures in different regions of the world within the last 50,000 years, since modern humans emerged Out of Africa.

Today, I want to focus on another important paper called "Clines, Clusters, and the Effect of Study Design on the Inference of Human Population Structure" by Noah A. Rosenberg et al. It's a refutation of one of a bit of sophistry by the more sophisticated members of the Race Does Not Exist crowd: the cline theory. Yeah, sure, people differ, but the variations change evenly across the face of the earth, so you can never define the boundaries of separate racial groups.

For example, Science Daily reported on a population genetics study that says:

"… geographic distance from East Africa along ancient colonization routes is an excellent predictor for the genetic diversity of present human populations, with those farther from Ethiopia being characterized by lower genetic variability... The loss of genetic diversity along colonization routes is smooth, with no obvious genetic discontinuity, thus suggesting that humans cannot be accurately classified in discrete ethnic groups or races on a genetic basis."

Rosenberg's new paper essentially quantifies my refutation last March of the pure clinal theory in The cline theory isn't a bad one as long as it's not pushed too far. I wrote:

Two fallacies are readily apparent in this statement. First, the whole argument is a little silly. You could walk from, say, Calais on the English Channel to Pusan in South Korea without dying of thirst. At either end of your vast journey, however, the people look quite different. In between you might run into, say, Boris Yeltsin, a blond man with features slightly reminiscent of East Asia, and other people of varying degrees of European and East Asian admixture. But, in the big picture, so what? Frenchmen and Koreans are still different and nobody would mistake one for the other.

Second, the geneticists' statement applies only "along colonization routes," and most possible directions were not major colonization routes. If you walk in the majority of directions, you will eventually fall into the ocean and drown. [Or die of thirst in the Sahara or of exposure in the Himalayas.] This reinforces the "obvious genetic discontinuity" that we see with our lying eyes. [More]

Rosenberg's paper is quite technical, but it shows definitively that both clines and clusters are important. If you assume five continental-scale races "corresponding to Africa, Eurasia (Europe, Middle East, and Central/South Asia), East Asia, Oceania, and the Americas," then, unsurprisingly, you find the clinal theory works well within a continental region or "cluster," but doesn't work well across the continental ("cluster") boundaries:

For population pairs from the same cluster, as geographic distance increases, genetic distance increases in a linear manner, consistent with a clinal population structure. However, for pairs from different clusters, genetic distance is generally larger than that between intracluster pairs that have the same geographic distance. For example, genetic distances for population pairs with one population in Eurasia and the other in East Asia are greater than those for pairs at equivalent geographic distance within Eurasia or within East Asia. Loosely speaking, it is these small discontinuous jumps in genetic distance—across oceans, the Himalayas, and the Sahara—that provide the basis for the ability of STRUCTURE to identify clusters that correspond to geographic regions.

Out of their 52 human populations, 49 fell distinctly into one continental-scale cluster or another. Only two followed the clinal model by being evenly mixed, and one, the pagan Kalash of the Hindu Kush, was unique:

Two exceptions to the pattern include the Hazara and Uygur populations, from Pakistan and western China, respectively, whose genetic distances scale continuously with geographic distance both for populations in Eurasia and for those in East Asia. These populations were evenly split across the clusters corresponding to Eurasia and East Asia, and thus, unlike most other populations, they do not reflect a discontinuous jump in genetic distance with geographic distance. Finally, a third population of interest in the plot is the Kalash population (of Pakistan), whose genetic distances to other populations are large at all geographic distances, illustrating the distinctiveness of the group as the only member of its own genetic cluster in some STRUCTURE analyses ...

The Kalash, like the Kafiristanis in "The Man Who Would Be King," claim to be descended from Alexander the Great's troops, and the Hazara say they are descendents of Genghis Khan's troops.

Excluding these three groups, Rosenberg found a strong correlation between geographic distance and genetic distance, as the clinal theory would predict, but also found that the three main kinds of barriers -- oceans, the Sahara, and the Himalayas -- "adds an equivalent amount of genetic distance as traveling approximately 3,100 km on the same side of the barrier." That's not a huge amount, but it's enough to make clusters as important a conceptual tool for thinking about human genetic diversity as clines.

My published articles are archived at -- Steve Sailer

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