Geographic isolation and human-assisted dispersal in land snails: a Mediterranean story of Helix borealis and its relatives (Gastropoda: Stylommatophora: Helicidae)

The Mediterranean basin is a major centre for land-snail diversity, with many localized endemics, but there are also species widely spread by humans. Both endemics and introductions can be found in the snail genus Helix, which comprises many large-bodied species used for human consumption in the past and present. The Mediterranean clade of Helix is currently distributed throughout this region, but the phylogenetic and biogeographic relationships among its forms from different parts of the basin remain enigmatic. The reasons include insufficient sampling, taxa with unclear taxonomy and a significant impact of human-assisted transport obscuring the natural distribution of phylogenetic lineages. We provide evidence that European and Anatolian populations of H. cincta and its relatives are not native to those regions, but originate from the northern Levant. These results have implications for taxonomy of the genus, but also for the understanding of its evolutionary history. We posit that the Mediterranean clade consists of four geographically separated groups, which diversified in Northern Africa, the Apennine Peninsula and Corsica, the Aegean and Greece, and the northern Levant. This geographic pattern has been subsequently blurred by multiple instances of human-assisted dispersal. However, revealing the founding populations with certainty requires thorough sampling in currently inaccessible countries