Differentiation and speciation in mountain streams: a case study in the caddisfly Rhyacophila aquitanica (Trichoptera)

Few studies have analysed the biogeography of mountain aquatic organisms, although this habitat provides stable conditions in which many species survived Pleistocene climatic oscillations, usually in the geographical vicinity of their present distribution ranges. The mountain caddisfly Rhyacophila aquitanica was selected as a model organism for this habitat type. Morphological measurements of genitalia and external characters of male individuals were obtained from almost the entire range of distribution of the species. Morphometric results were analysed by cluster analysis and multivariate statistics. Important differences were discovered among three population groups of R. aquitanica inhabiting different European mountain ranges: (i) mountain ranges north-west of the Alps (Massif Central, Vosges, Schwarzwald and Fribourg); (ii) the southern Alps (Lombardia and Carinthia); and (iii) the western part of the southern Carpathians. This divergence suggests a long-term isolation among these groups, which presumably took place long before the last Pleistocene glaciation, with no secondary contact among these populations. The differentiation centres of the southern Alps and Carpathian groups may have been mostly homotopic to their actual ranges, whereas the western group must have been distributed in the areas west or north-west of the Alps with secondary expansions and disjunctions.