Palaeogeography or sexual selection: Which factors promoted cretan land snail radiations?

Abstract

The high land snail diversity of Crete is the result of a few radiations. It has been suggested that these radiations were triggered by the fragmentation of Crete in the Neogene. Contrary to the predictions of this model, the ranges of the endemic species are not clustered and their diversity is not higher in the areas of the Neogene palaeo-islands. We investigated the radiation of the helicoid genus Xerocrassa in detail. The asymmetry between the range sizes of sister species and clades of Xerocrassa indicates that peripatric speciation was the predominant speciation mode. Coalescent simulations show that the differences in the genitalia of the Xerocrassa species cannot be explained by genetic drift. They can also not be explained by natural selection against hybrids, because they are not larger between geographically overlapping groups than between allopatric groups. The evolution of differences in flagellum length is concentrated towards the tips of the tree indicating that sexual selection might have facilitated the radiation. If speciation is driven by sexual selection, niches may remain conserved and non-adaptive radiation may result.