Commentary on Sokal's "Ancient Movement Patterns Determine Modern Genetic Variances in Europe" (1991)

Correction:

Due to an error at the printer, the original article page range (551–552) was incorrect. The correct pagination is 553–554 and is reflected in this updated article. Click here for the corrected PDF.

In his work on quantitative systematics, Robert Reuven Sokal introduced the idea of spatialization in the late 1970s. After his studies on metapopulations of red flour beetles, house-flies, and cottonwood gall (Bell 2012), he became interested in human metapopulations in the 1980s. This genre had been somewhat neglected, probably because of the shadow cast by the outcomes of eugenics in the earlier half of the twentieth century. As an Austrian Jew who took refuge in China to escape Nazi eugenics, Bob Sokal was perhaps the right man to venture into the field of biological anthropology, armed with the theoretical tools of systematic zoology, in which beast and Man are equal.

To analyze the allele or haplotypic frequencies of classic genetic markers, the approach then emerging was to consider these frequencies as samples of surfaces of biological variables on the map, then represented in two dimensions by their so-called principal components (Menozzi et al. 1978). To understand the hypothetical cause(s) that generated geographical patterns of human population, it was (and is) natural to bring in prehistoric and historic sources that provide information on population biology. Based on the similarity between the geographical pattern of a large fraction of allele frequencies and the Neolithic expansion from the Levant, Ammerman and Cavalli-Sforza (1984), as we know, attributed the founding impact of the current genetic structure of European populations to the latter. But the history of European genetic structure was shaped not only by the long, quiet flow of the Neolithic, but also by subsequent "partial migration to, expanding into, settling in, conquering, attacking the area militarily without settling there, naval attacks, etc." (Sokal 1991: 592). In this article, Bob Sokal innovates on two points: Instead of focusing on allele frequencies, understood only as sampled values of the continuum at different localities, he turned to local variations in these frequencies, as estimated in each square of a grid. In theory, it seems obvious that localities that came to be of historical importance—as expressed by the movements of populations in these localities either as places of departure (sources) or places of arrival (targets)—would have high within-locality genetic variance, while localities that did not have this importance would have low or nil genetic variance. Therefore, a positive correlation would be expected between the intensity of population movements within localities and their respective genetic variances. The same would be true for genetic similarities between localities, which should be correlated with the directional intensity of exchanges between them (as sources or as targets).

The second innovation in this article by Bob Sokal is the creation of an ethnohistorical database used to quantify the multitude of population movements in localities on the map. This ethnohistorical database covers the same geographic reference frame as the genetic data. It reflects population movements after the Neolithic expansion from 2000 BC to 1970 AD, that is to say, after the founding impact that shaped the structure of European genetics. (The ethnohistorical database is still downloadable at http://life.bio.sunysb.edu/ee/msr/Ethno/.) The results of this vast work show, apparently and despite the article's title, a very modest influence of historical movements on the modern genetic structure, more modest than Gustave Malecot's "isolation by distance," expressing the limited geographical mobility of individuals in the populations.

But the scientific requirement to bring out the impact of population movements in the last 4,000 years on today's population structure, both conceptually— quantitative and now geographical systematics—and in terms of data, was taken by Bob Sokal to a level that became a model for many subsequent studies, regarding the origins of current genetic diversity and its correlates in several world regions (Barbujani and Pilastro 1993; Calafell et al. 1996; Harich et al. 2002; Hunley et al. 2008) and in spatial analysis of human variation (Barbujani and...