Differentiation and Population Structure of Idyla bicristata (Gastropoda: Clausiliidae) on Euboea, Greece

ABSTRACT Idyla bicristata is a highly variable rock‐dwelling door snail species living on limestone rocks around the Aegean Sea. The island of Euboea and the adjacent Greek mainland are a hotspot for morphological variability in I. bicristata , resulting in the various forms being named. A population genetic analysis of 23 populations of I. bicristata based on SNPs from more than 10,000 restriction‐site associated DNA markers revealed two population groups that occur both on Euboea and the adjacent Greek mainland. Their distribution corresponds to two tectonic zones with different geologies. The populations of the southern group are more strongly differentiated and are paraphyletic with regard to the northern population group, which diverged via a star‐like radiation. The relationships between these groups suggest that Euboea was colonised from the south. Shell size, sculpture, and the development of a whitish surface layer depend on environmental parameters such as precipitation. Most of the previously named morphological forms do not represent distinct population groups, but rather adaptations to the local environmental conditions or even non‐genetic modifications.