The internal genetic variability studies carried out to date on the Basque population using conventional markers indicate a series of factors which make analysis difficult. Among these factors is the lack of rigour in some studies as regards specifying the origin of samples, the lack of match as regards the polymorphisms analysed in different studies and methodological problems in some analyses. @@ The aim of this study is to obtain an overall configuration of the internal genetic variability of the Basque population in the Iberian Peninsula via a uniform sampling system and method of analysis throughout the territory involved. To do this we carried out a genetic analysis of 18 population series: one province-wide sample (in Navarre) and 17 samples from natural districts in the provinces of Alava, Guipuzcoa, Navarre and Biscay. The study was carried out by analysing eight highly informative DNA genetic markers in a sample of 845 autochthonous Basque individuals. @@ The results obtained indicate genetic heterogeneity within the Basque population, explained equally between districts and provinces on the one hand and between provinces and the Basque Country as a whole on the other. The evaluation of gene flow in the area studied at district level indicates a trend, with the Goierri district of Guipuzcoa at one end and a group of districts in which more major gene flow has taken place at the other. These districts are Durango, Deba and Montaña Alavesa, which are characterised by standing on intra- or inter-provincial communication routes, and Iruñea and Llanada, which contain the provincial capitals of Iruñea and Gasteiz and are therefore meeting points for people from other districts. @@ The fact that there is greater similarity between the districts of a single province than between districts of different provinces, the inter-province genetic heterogeneity observed and the intra-province homogeneity found in Alava, Guipuzcoa and Navarre gives genetic consistency to the existence of these provinces. This can be interpreted as a residue of an ancient diversification which appears not to have been diluted by the effects of gene flow; or as a reflection of cultural manifestations, particularly language, for which similar frontiers exists (though these have not remained unchanged over time). @@ The absence of significant genetic differences between the Basque-speaking and Spanish-speaking provinces of the Basque country leads us to conclude that the decline in the use of the Basque language from its peak in the 10th century may be due to cultural replacement or a process of acculturation which has gradually reduced the area where Basque is spoken, rather than to any process of migration of allochthonous populations, which would have had different effects in different districts. @@ The hypothesis suggested in earlier studies that the watershed could be the genetic frontier between the two main groups of Basques, with a greater population mix in the south, does not seem to be supported by the results of this study. In topological and tree diagrams of multivariate analyses it is the provinces of Navarre and, especially, Biscay, which are closest to European series as a whole, while Alava is furthest away from those series. @@ These results point to a strong correlation between genetics, geography and language in the structuring of gene frequencies. Thus, the importance given to the local community as the basis for social organisation seems to entail the establishing of small units with gene flow basically between adjacent or neighbouring districts, but without reaching any genetic homogenisation in the area analysed in this stud
