Influence of Changes in Political Barriers and of Geographic Distance on Kinship Inferred from Surnames and Migration Data in Olivenza (Spain) and Surrounding Portuguese Areas

The existing relationship between human populations is a function of their migratory and genetic exchange which will be inversely proportional to the distance separating them. The effect of geographic distance on population structure may be estimated by means of isonymic methods which use information on the surnames present in a territory as an approximation to the distribution of allele frequencies. The objective of this study is to analyse whether the modification in 1801 of the political border in an area surrounding the town of Olivenza, which experienced a change of sovereignty from Portugal to Spain, has had noticeable influence on the migration pattern and isolation by distance in that region. For this purpose data from marriage records of Olivenza and the neighbouring Portuguese municipalities of Alandroal, Juromenha, Elvas, Vila Boim (and Terrugem), Terena, Monsaraz, and Vila Viçosa were analysed. Two periods were considered in order to analyse the population structure: one prior to the change of domain (1775–1801), another after it (1802–1825), for which rates of diversity and inbreeding coefficients were determined. The results obtained show that following the border modification, the migration matrices changed differently according to sex, therefore altering the relationship between the various localities of the territory. In Olivenza inbreeding declined slightly and surnames became more heterogeneous. Moreover, following the change of domain the isolation by distance models illustrate a temporal reduction in the relative weight of geographical distance on inter-population kinship. The political border acted as a factor in population differentiation in the Iberian Peninsula (Spain and Portugal)