The species delimitation problem applied to the Agabus bipustulatus complex (Coleoptera, Dytiscidae) in north Scandinavia

Agabus bipustulatus (Linnaeus) is one of the most common aquatic beetles in Europe. Two species have been traditionally recognized within the Palaearc tic Agabus bipustulatus complex (Coleoptera, Dytiscidae) in Scandinavia: a lowland form A. bipustulatus and a high-altitude form A. solieri Aube. The specific status of solieri has been debated for more than a century but no quantitative investigation has been made to evaluate the status of this tax on. In this study we show that there is no clear-cut delimitation between t he two forms, either morphological or genetic, across an altitudinal gradie nt in north Scandinavia. Morphological differences between 22 populations w ere analysed separately for each sex with both thin-plate splines relative warp analysis and 'classical-length' morphometrics. Genetic variation at fi ve polymorphic enzyme loci was analysed among seven populations. The morpho logical studies showed gradual variation correlated with altitude, in parti cular in the character that is traditionally used to separate solieri and b ipustulatus, and in both the beetles' morphometric size and the lateral wid th of the metasternal plate, which is connected to flight capacity. The gen etic study indicates that the alpha -Gpdh enzyme locus, which is involved i n the transfer of energy to the night muscles, is evidently subject to dire ctional selection. Only minor population differences were observed without this system. Subdivision was found in some populations and was probably cau sed by migration from outside or within the local population. The overall c onclusion is that there is no clear-cut species delimitation between A. bip ustulatus and A. solieri in north Scandinavia. This indicates that A. solie ri is a cold-adapted altitudinal form of the variable A. bipustulatus; addi tional support for this is the finding of solieri 'look alikes' in cold spr ings in areas normally inhabited by bipustulatus. (C) 2001 The Linnean Soci ety of London.