Captured in 'continental archipelago': phylogenetic and environmental framework of cranial variation in the European snow vole

Fragmented landscapes entail important consequences for the evolution of the species confined to them. Isolation of population fragments accelerates selection for narrow local conditions and facilitates morphological divergence. Throughout its range, the European snow vole Chionomys nivalis is restricted to fractured, rocky substrate in mountain regions, which is naturally fragmented into 'continental archipelagos'. Consequently, its extensive morphological and genetic diversity was categorized into about 20 traditional subspecies and at least eight allopatric phylogenetic lineages. In this study, we aimed toward an integrative understanding of cranial variation throughout the European snow vole range. We analyzed seven linear cranial variables on 326 adult skulls from 27 populations belonging to eight phylogenetic lineages. We confirmed significant variation among the fragmented populations but retrieved little meaningful patterning in morphometric variability. Phenetic distances among populations were not related to the phylogenetic architecture of the species, and traditional subspecies were at odds with morphologically diagnosable populations. The lack of an association between morphometric and geographic distances argued against isolation by distance. Furthermore, mean size did not correlate with climatic variables. Morphological principal components 2 and 3 (loaded by the interorbital width and length of neurocranium, respectively) correlated significantly with geographic coordinates and climatic variables. Shape variables discriminated between the abulensis and nivalis phylogroups, and the European and Asiatic populations, but the largest phylogroups (nivalis and malyi) showed high interpopulation heterogeneity and classification accuracy was low. We suggest that cranial shape or size do not incorporate a signal that is strong enough to be of reliable use for subspecific taxonomy. Skull is seemingly prone to vary according to narrow local conditions, which distorts the underlying phylogenetic signal. A small-scale approach, with detailed knowledge of environmental parameters within each habitat fragment, might be more appropriate for a species whose range is actually a continental archipelago.