Geographic morphological variation in parapatric Western Palearctic tree frogs, Hyla arborea and Hyla savignyi: are related species similarly affected by climatic conditions?

Tree frogs Hyla arborea and Hyla savignyi are similar, closely-related species distributed in Europe and the Middle East. We investigated geographic variation in body shape within and between these species, and tested its relationships to macroclimatic conditions. We used morphometric distances (based on size corrected external measurements) to construct phenetic trees (unweighted pair-group method of arithmetical averages, Neighbour-joining), and to test correlations between morphology, geography, and climate by the partial Mantel test. Regardless of their specific affiliation, the parapatric populations of both species from the eastern Mediterranean, where they occupy comparable habitats, are closer to each other in morphospace than to conspecific populations from distal regions. This local interspecific similarity is probably driven by the common response to environment, expressed here as macroclimatic conditions. In support, the geographically close but ecologically vicariant populations of both species from the Caucasus region differ quite substantially in body shape. We suggest that climate-provoked phenotypic variation in closely-related parapatric species should be taken into account as a potential complication to character displacement in morphology. Contrariwise, morphological diversification between related species or their populations could be enhanced by habitat shifts resulting in occupation of different environmental space. (c) 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 539-556.