Gene Frequency Clines in Europe: Demic Diffusion or Natural Selection?

Article excerpt

According to the classic view of Childe (see, for instance, 1958), agriculture was part of the Neolithic Revolution, a quantum leap in human adaptation, producing a much higher population density than foraging had allowed. Originating in the Fertile Crescent of the Near East, expanding agricultural peoples spread through Europe in a northwesterly direction, ultimately replacing the Mesolithic foraging populations.

Ammerman and Cavalli-Sforza (1971) quantified this spread on the basis of carbon-14 dates available at that time and found that it could be described as a wave of advance (see Fisher 1937) of approximately 25 km per generation. Cavalli-Sforza and colleagues (for instance, Menozzi et al. 1978) argued that the Neolithic was spread by an actual colonizing movement of farmers (a la Childe), in a process they called demic diffusion. The dynamic responsible for this population expansion was the greatly increased growth rate of farming populations and the much higher carrying capacities allowed by agriculture. Rapidly growing farming populations budded and colonized relatively empty foragers' territories, quickly assimilating and absorbing the few foragers into their groups.

The significance of such demic diffusion for the genetic structure of Europe, according to Cavalli-Sforza's group (and also, to some extent, Sokal and colleagues [see Sokal & Menozzi 1982; Sokal et al. 1991]), was that modem clinal patterns observed in analyses of gene frequencies are apparently congruent with the direction of spread of agriculture from the southeast to the northwest. Thus, given the archaeological evidence for a wave of advance of agriculture, and a theory that assigns much greater population growth potential to farmers, the clines are seen as migrations of Neolithic people as they spread from the centre of domestication in the Near East.

This argument follows Childe in presuming a major technological advantage of early European agriculture over 'aboriginal' foraging economies. It also clearly depends on the archaeologically-derived rate and pattern of spread of agriculture. If these crucial constraints on the model are incorrect, the explanation of gene frequency clines becomes suspect. As Weiss (1988) has pointed out, models can be constructed to fit any data given no constraints on the parameter values. A cline is, after all, only a correlation and correlation is not causation. The great advantage of the model proposed by Menozzi et al. (1978) appeared to be the known facts of agricultural spread, which gave credence to the evolutionary mechanism (see Fix 1979). This evidence made the demic diffusion model convincing (and, indeed, I [Fix 1979] and Weiss [1988] were both convinced).

An examination of the recent archaeological literature, however, suggests that the 'facts' are not as clear as once thought (see Barker 1985; Dennell 1992; Gregg 1988; Zvelebil 1986). Particularly relevant to the demic diffusion argument is the lack of evidence for rapid population expansion of many early Neolithic populations, and the evidence for relatively large, complex Mesolithic populations. These findings suggest that alternative explanations for clines should be investigated. The goal of this article is to present such an alternative to the demic diffusion model for gene frequency clines in Europe.

The demic diffusion model

The basic outlines of the demic diffusion model were presented by Ammerman and Cavalli-Sforza (1971) and extended in several subsequent publications (see Ammerman & Cavalli-Sforza 1984; Rendine et al. 1986). Further extensions of the idea to other human populations were made by Cavalli-Sforza et al. (1993; 1994). The core of the hypothesis is that agriculture expanded, not by the diffusion of techniques, tools and cultigens to resident populations, but by the actual movement of people bearing agriculture to new areas. The cause of this expansion was population growth of farmers, leading to the colonization of new territories and the displacement of the resident foragers (Ammerman & Cavalli-Sforza 1971: 686). …