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Geographical disjunction and environmental conditions drive intraspecific differentiation in the chalk-hill blue butterfly


 Drivers of evolution are often related to geographical isolation and/or diverging environmental conditions. Spatial variation in neutral genetic markers mostly reflects past geographical isolation, i.e. long-lasting allopatry, whereas morphology is often driven by local environmental conditions, resulting in more rapid evolution. In Europe, most thermophilic species persisted during the past glacial periods in geographically disjunct refugia, representing long-lasting isolates, frequently with diverging environmental conditions. This situation has driven the evolution of intraspecific signatures in species. Here, we analysed wing shape and wing pigmentation of the chalk-hill blue butterfly, Polyommatus coridon, across its entire distribution range restricted to the western Palaearctic. In addition, we compiled abiotic environmental parameters for each sampling site. Wing colour patterns differentiated a western and an eastern lineage. These lineages might represent two main Pleistocene refugia and differentiation centres, one located on the Italian Peninsula and the other in the Balkan region. The two lineages showed evidence of hybridization across Central Europe, from the Alps and across Germany. The intraspecific differentiation was strongest in the width of the brown band on the outer margin of the wings. The morphological structures obtained are in line with genetic signatures found in previous studies, but the latter are more fine-grained. Current environmental conditions, such as mean temperatures, were only marginally correlated with colour patterns. Our study underlines that Pleistocene range shifts, often resulting in allopatric isolation, shape intraspecific phenotypic structures within species; that pigmentation responds in a more sensitive manner to spatial disjunction than wing shape; and that morphometric and genetic structures in P. coridon provide concordant patterns and thus support identical biogeographical conclusions.