Gene frequency clines produced by kin-structured founder effects

Two mechanisms are commonly invoked to explain geographic gradients in gene frequencies (clines): gene flow and natural selection. Thus gene frequency clines in Europe have been attributed to gene flow because expanding Neolithic farmers from the Near East spread and absorbed resident foragers, a process Cavalli-Sforza and his colleagues labeled demic diffusion. Alternatively, gradients in natural selection can produce the same genetic pattern. A third mechanism to explain European clines has recently been proposed by Barbujani et al. (1995). They found that European allele frequencies were consistent with distributions generated by a simple model of total replacement of foragers by expanding farmers. Clines were produced by repeated founder effects in colonizing farmer groups. Here, I present a simulation model using different population parameters to test the generality of the Barbujani model. Results of the simulation show that steep clines (measured by spatial autocorrelation statistics) can be produced by kin-structured founder effects. The implication of these findings is that several genetic mechanisms are consistent with the allele distributions in Europe. To discriminate among these alternatives, other information, such as better archeological and demographic characterization of interactions between farmers and foragers, is required. In any case, the mere presence of genetic clines does not necessarily validate the demic diffusion model.