From surnames to linguistic and genetic diversity: five centuries of internal migrations in Spain.

In a previous study concerning 33,753 single Spanish surnames (considered as tokens) occurring 51,419,788 times we have shown that the present-day geography of contemporary surname variability in Spain still corresponds to the political geography of the country at the end of the Middle Ages. Here we reprocess the same database, by clustering surnames with Self-Organizing Maps (SOMs) according to their geographic distribution, to identify the monophyletic surnames showing a geo-historical origin in one of the 47 provinces of continental Spain. They are 25,714, and they occur 12,348,109 times, meaning that about 75% of the Spanish population bears a surname that had a polyphyletic origin. From monophyletic surnames we compute migration matrices accounting for the internal migrations that took place since five centuries ago, when Spanish surnames started to be patrilineally inherited. The mono/ polyphyletic classification we obtain fits ancient census data and is compatible with published molecular diversity of the Y-chromosomes associated to selected Spanish surnames. Monophyletic surnames indicate that i) the provinces exhibiting a higher percentage of autochthonous surnames are also ii) those from which emigration corresponds to a local isolation-by-distance model of diffusion and iii) those that attracted a lower number of immigrants. These are also the provinces where languages other than Castilian are spoken. We suggest that demographic stability explains linguistic resilience, as people prefer to move to areas in which the linguistic variety is more similar to their own. So far the reciprocal influence of migration and language has been investigated at local scales, here we outline how to investigate it at national scales and for time-depths of centuries.