Isonymy and isolation by distance in Italy

The isonymy structure of Italy was studied using the surname distribution o f 5,043,580 private telephone users selected from a 1996 commercial CD-ROM that contains all 24 million users in the country. The users were distribut ed in 123 towns selected on a geographic basis. The 123 towns were either o n the main communication roads of the country or at the ends of such roads. The shortest distance between nearest neighbor towns was 5.3 km (Carrara a nd Massa), and the largest distance was 1,136 km (Aosta and Castrignano del Capo). The number of different surnames found in the whole analysis was 21 5,623. Lasker's distance, the negative value of the logarithm of random iso nymy between localities, was linearly and significantly correlated with the logarithm of geographic distance, with r = 0.63 +/- 0.008. A dendrogram wa s built from the matrix of isonymy distances, using UPGMA. It separates the Italian towns into 5 main clusters: 1 in the southern portion of the count ry, a second cluster toward the center, and 3 in the northern area of Italy . Within each cluster small subclusters with specific geographic distributi ons could be related to regional borders. Comparisons with the results of a previous analysis of Switzerland and Germany's structures are given. From the present analysis isolation by distance emerges clearly, although it is less strong than in Switzerland and stronger than in Germany. The random co mponent of inbreeding estimated from isonymy indicates that the southern ar ea of Italy is on average more inbred than the northern area. In fact, the heterogeneity is greater in the northern area, particularly in the plain of the Po River, than anywhere else in Italy.