Biological Journal of the Linnean Society, 2003, 79, 611–628. With 9 figures © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 79, 611–628 611 Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 2003? 2003 79? 611628 Original Article SPECIATION IN LAND SNAILSM. PFENNINGER ET AL. *Corresponding author. E-mail: Pfenninger@zoology.uni- frankfurt.de Evidence for ecological speciation in the sister species Candidula unifasciata (Poiret, 1801) and C. rugosiuscula (Michaud, 1831) (Helicellinae, Gastropoda) MARKUS PFENNINGER 1 *, ANNEKATRIN EPPENSTEIN 1 and FRÉDÉRIC MAGNIN 2 1 Abteilung Ökologie und Evolution, Zoologisches Institut der J.W.Goethe-Universität, Bio-Campus Siesmayerstraße, D-60054 Frankfurt/Main 2 IMEP UMR 6116 Case 461, Faculté des Sciences et Techniques de St. Jerome, 13997 Marseille Cedex 20, France Received 20 October 2002; accepted for publication 29 January 2003 This paper studies the speciation of two land snail species in south-east France. By using two mitochondrial and one nuclear gene, the species are shown to be sister species within the larger clade of western European Candidula spe- cies. The species never occurred syntopically, but a narrow contact zone population was identified. Recent range expansions inferred from phylogeographical methods indicated that the present day distributions are not limited by the colonization capacity of the species. Analysis of environmental variables suggested distribution along an ecotone. The gastropod community is also correlated to this gradient. Morphometric shell analysis showed that the diver- gence between the sister species is a result of desiccation-resistant shell characters. We therefore conclude that the cause for the ecological speciation is most probably the establishment of the Mediterranean climate ª 3.2 mya. A model of ecological speciation with a moving ecotone is put forward. © 2003 The Linnean Society of London, Bio- logical Journal of the Linnean Society, 2003, 79, 611–628. ADDITIONAL KEYWORDS: 16S rDNA – Candidula phylogeny – ecological selection – ITS-1 INTRODUCTION The debate about the mechanisms or modes of speci- ation is one of the most fundamental in evolutionary biology. While allopatric speciation (Mayr, 1963) was for many years the only accepted mode, the focus has changed in recent years from the geographical arrangement of populations to the forces that drive the divergence (Howard, 1998). Schluter (2001) clas- sified four modes of speciation according to their initial cause: ecological speciation, speciation by diver- gence under uniform selection, speciation by genetic drift and speciation by chromosomal reorganization. These processes are not restricted to a particular geo- graphical scenario and may take place in sym-, para- or allopatry. The inference of the most likely mode of speciation for a particular species pair should therefore include a detailed understanding of their phylogenetic relation- ships, geographical distribution and natural history (Howard, 1998). However, species are usually not homogenous entities, but are spatially structured and occur over a range of more or less ecologically different environments. Furthermore, present day distributions may not reflect distributions in the past. A population- based approach, including as many populations from the taxa in question as possible, and utilizing phylo- geographical and ecological techniques is thus indis- pensable to study speciation. Land snails are ideal organisms to study historic evolutionary processes in species with limited dis- persal. Although a few land snails have attained a worldwide distribution via anthropogenic displace- ment, the range of most land snails is usually quite restricted because of their poor dispersal ability and particular habitat requirements (Cowie, 1984; Baur & Baur, 1993; Pfenninger, Bahl & Streit, 1996). These characteristics prevent land snails from actively escaping changing ecological conditions. Low dispersal