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Aim Disentangling the roles of the interacting processes that shape species' ranges requires independent measurements of dispersal, physiological traits and habitat use. Multifaceted approaches of range determinants are, however, still rare, despite the widespread recognition that correlative modelling approaches alone are not sufficient to understand and predict species' distributions. Here, we combined genetic, distributional and physiological data to reveal the processes that cause the disjunct distribution of the groundwater isopod Proasellus valdensis in isolated Alpine mountains previously covered by Pleistocene glaciers. Location The Alps and Jura Mountains, France. Methods Phylogenetic/phylogeographical methods based on mitochondrial and nuclear genes were used to test for recent dispersal between mountains. A logistic regression on presence-absence data was performed to quantify variation in the probability of occurrence with temperature. Variation in survival and respiration over a range of temperatures was measured within four populations to test for a causal effect of temperature on species distribution. Results Despite the disjunct distribution, genetic analyses supported recent dispersal between mountains, as indicated by weak divergence among sequences of cytochrome c oxidase subunit I ( COI), a single haplotype network showing no spatial structuring, and a small proportion of molecular variance distributed between mountains. The probability of occurrence of P. valdensis decreased significantly with increasing temperature, although physiological experiments indicated that occurrence in warmer habitats was probably restricted by thermally dependent biotic interactions rather than by temperature itself. All populations maintained a high survival rate over a wide range of temperatures (3-15 °C), with a weak but detectable tendency for local adaptation. Main conclusions Combining phylogeographical, physiological and habitat modelling methods reveals the interacting processes that drive range dynamics. A broad thermal tolerance helps P. valdensis to colonize vacant habitats during the onset of glacial melting, but range fragmentation and local adaptation, leading to thermal niche narrowing, proceed during warmer interglacials as biotic interactions progressively intensify. [ABSTRACT FROM AUTHOR]