Inferring the impact of linguistic boundaries on population differentiation: application to the Afro-Asiatic-Indo-European case

We present here a quantitative way to assess the impact of language-family boundaries on population differentiation and to evaluate the homogeneity of the genetic processes along these boundaries. Our estimator (delta a) of t he impact of the boundary is based on an isolation by distance (IBD) model and measures the added genetic distance between populations located on diff erent sides of the boundary. We compare this statistic with another estimat or of group differentiation (F-CT) computed under an analysis of variance f ramework that does not assume any particular spatial structure of the popul ations. Monte Carlo simulations are used to study the behaviour of these st atistics under a two-dimensional stepping-stone model. Simulations show tha t F-CT can suggest the existence of a frontier when populations only differ because of IBD. This spurious behaviour is much less frequent for the delt a a statistic. However, the large variance associated with the delta a stat istic, and the fact that it should only be computed in the presence of IBD, may limit the use of this statistic. Overall, the origin and the effect of the boundary is best; understood by comparing different statistics and by testing for the presence of IBD on each side of the boundary as well as acr oss the boundary. We illustrate our approach by examining the boundary betw een Afro-Asiatic and Indo-European populations. These populations are globa lly genetically differentiated, but the effect of the linguistic boundary o n gene flow seems geographically very heterogeneous. This boundary appears to be the result of a secondary contact between two differentiation centres rather than an enhancer of population differentiation.