Genetic differentiation between natural populations of Drosophila subobscura in the Western Mediterranean Area with respect to chromosomal variation

Abstract

Summary The chromosomal arrangement frequencies of 27 populations of Drosophila subobscura from the western Mediterranean region have been compared. To evaluate the relationships between these populations, factorial analysis of correspondences (B1973) and a distance proposed by P (1974 a) have been used. The general clines present throughout the distribution of the species were also detected in the western Mediterranean area. Distinct chromosomal polymorphism was found in three areas : Tunisia, continental Europe and the whole of Sicily and Sardinia. Smaller islands were subject to stronger foreign influences. Iviza, Ponza and Ventotene showed small differences with the nearest continental populations. Ustica was rather similar to Sicily, its nearest major area ; however, Lipari, in spite of its vicinity to Sicily, showed greater similarity to the more distant continental Italian populations. The Corsican population seemed to be influenced by all its surrounding populations. The observed differences appear to be correctly interpreted as the consequence of adaptive selection (expressed in latitudinal clines) interacting with historical factors related to physiographical conditions which determine the degree of isolation between populations. Also, the size of the concerned regions seemed to be important : two of the three well differentiated areas were continental and the third included the two larger islands, Sicily and Sardinia. Corsica, next in size, showed a high frequency of a generally rare arrangement, 0 if genetic drift were a main factor in the differentiation of these populations, the reverse situation would be true.