Deep structure, long-distance migration and admixture in the colour polymorphic land snail Cepaea nemoralis

While snails of the genus Cepaea have historically been important in studying colour polymorphism an ongoing issue is that there is a lack of knowledge of the underlying genetics of the polymorphism, as well as an absence of genomic data to put findings in context. We therefore used phylogenomic methods to begin to investigate the post-glacial history of Cepaea nemoralis, with a long-term aim to understand the roles that selection and drift have in determining both European-wide and local patterns of colour polymorphism. By combining prior and new mitochondrial DNA data from over 1500 individuals with ddRAD genomic data from representative individuals across Europe, we show that patterns of differentiation are primarily due to multiple deeply diverged populations of snails. Minimally, there is a widespread Central European population and additional diverged groups in Northern Spain, the Pyrenees, as well as likely Italy and South Eastern Europe. The genomic analysis showed that the pres...