Patterns and processes in a non‐adaptive radiation: Alopia (Gastropoda, Clausiliidae) in the Bucegi Mountains

Abstract

AbstractWe studied the door‐snail genus Alopia in the Southern Carpathians in Romania to better understand non‐adaptive radiations and the processes that determine their course. Alopia in the Bucegi Mountains offers the opportunity to study all stages of a radiation within a few kilometres. The species (as defined by the differential fitness species concept) in the most advanced stage of the radiation differs from other species in the genitalia and co‐occurs with other species. The least advanced stages are genetically differentiated clusters of populations that are geographically separated from other clusters but are not yet morphologically differentiated. Differentiation does not increase with a constant rate as shown by a lineage that was separated early in the evolution of the genus but fused with other taxa upon secondary contact. Since non‐adaptive radiation cannot be accelerated by divergent natural selection beyond the slow pace of speciation that is possible by genetic drift alone, sexual selection plays a crucial role in non‐adaptive radiations. This is supported by the differences in the genitalia found in the most advanced stage of the speciation which indicate that the speciation process was accelerated by a co‐evolutionary arms race resulting in the elongation of spermatophore‐producing and spermatophore‐receiving organs. Another process that facilitates non‐adaptive radiation is long‐distance dispersal that results in geographically isolated populations that can differentiate without gene flow and that have a higher likelihood of speciation due to the founder event. Several taxa that were considered distinct species until now, but fuse upon contact should better be classified as subspecies of a polytypic species.