Fluvial range expansion, allopatry, and parallel evolution in a Danubian snail lineage (Neritidae: Theodoxus)

The snails Theodoxus danubialis and Theodoxus prevostianus form a single clade native to freshwaters of south-eastern Europe whose inter- and intraspecific relationships remain unresolved. The present study utilized a phylogeographical approach to clarify the relationship of these species as well as to reconstruct the evolutionary and demographic history of populations in the western portion of their range. Phylogenetic, population genetic, and nested clade analyses reveal a clade that has distributed itself upriver from a lower Danube River source population and become genetically distinct primarily through range expansion and localized allopatric divergence. Notably, this geographical pattern is replicated phylogenetically in the form of two cytochrome c oxidase subunit I (CO I) lineages that are present simultaneously in individual snails. Haplotypes from polymorphic individuals form two distinct clades, both of which show phylogenetic and nucleotide substitution patterns consistent with a mitochondrial origin, and whose common ancestor must have occurred in a lower Danube source population. Separated allopatrically from their Danubian relatives, populations of T. danubialis in northern Italy have also undergone substantial range expansion, much more recently than Danube watershed lineages. In addition to repeated patterns of range expansion, parallelism is found in T. prevostianus , which is shown to be a nonmonophyletic taxon of remarkable morphological and ecological similarity.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 603–617.