A genetic reconstruction of the history of the population of the Iberian Peninsula

Abstract

The genetic patterns detectable in human populations of the Iberian Peninsula are shown by means of 'synthetic genetic maps', i.e. geographic maps of the highest principal components (PC) of gene frequencies. This method of analysis separates independent patterns of the genetic landscape, which hopefully represents different, major evolutionary events of the past. Among these are clines established by ancient important migrations, and local differentiations of populations due to barriers responsible for relative isolation. Only events of some magnitude from a demographic point of view, involving populations having initially definite genetic differences are detectable by the method. For this to be true, the genetic consequences of these events must not have been entirely smoothed out by later, prolonged genetic exchange between neighbours; but simulations have shown that long clines produced by major migrations can be rather stable in time. The first synthetic map, corresponding to the first PC, shows that the major difference in the Iberian Peninsula is that between people originally of Basque and non-Basque descent. The recession in time of the boundaries of the Basque-speaking area seems correlated with the progressive genetic dilution of the Basque genotype in modern populations, as we move away from the centre of the Basque area. Clearly there must have been a close relationship in the progressive loss of the Basque language and increasing genetic admixture with neighbours. Most probably, Basques represent descendants of Paleolithic and/or Mesolithic populations and non-Basques later arrivals, beginning with the Neolithic. The second synthetic map is correlated with early Neolithic infiltrations from the eastern edge of the Pyrenees. It has been shown by archaeologists that, in some areas, early Neolithics lived side by side at overlapping dates with well developed Mesolithics. The demographic impact of Neolithic farmers versus Mesolithics, and therefore their genetic influence was thus less important in the Iberian Peninsula than in Central Europe. The third synthetic map shows a correlation with the linguistic and historical duality between the Atlantic and Mediterranean fringes, which developed in the first millennium B.C. and was probably determined, to some extent, by infiltrations through the Pyrenees of the Urnfield cultural elements as well as by several other later events.