Invading a refugium: post glacial replacement of the ancestral lineage of a Nymphalid butterfly in the West Mediterranean

The power of a fixed scenario was compared to that of a dynamic scenario, in order to explain the distribution pattern of two well established subspecies (morphotypes) of the model butterfly species Maniola jurtina in the central Mediterranean. Samples were collected from a transect of 21 sites along the western side of the Italian Peninsula as well as in Sicily, North Africa and some Italian islets in the Tyrrhenian sea. Samples from the Balkans Peninsula have been added for comparison. Geometric morphometrics, suggested as a reliable marker to identify hybrid individuals, were applied to 150 male genitalia. Their shape was analysed by the means of the partial least square (PLS) discriminant analysis and then modelled through geographic information system (GIS) spatial analyses. The timing of invasion was reconstructed by comparing sea-level changes with the recent isobaths both on the mainland and on islands. The occurrence of the eastern morphotype on the Italian Peninsula and of the western morphotype in North Africa and Sicily was confirmed. However, we found intermediate populations at the tip of the Italian Peninsula and on the islands of Ischia and Capri. No intermediate populations were found in Sicily. The fixed scenario is unlikely, since a dispersal of the western morphotype from Sicily to the distant islands of Ischia and Capri might be hypothesized. A more parsimonious hypothesis minimises dispersal across the sea barriers. It assumes the ancestral presence over the entire study area of the western morphotype, which was later replaced on the Italian mainland but maintained on the islands. These rapid movements could drastically modify European biogeographic patterns.