The history of repeated northern glacial cycling and southern climatic stability has long dominated explanations for how genetic diversity is distributed within temperate species in Eurasia and North America. However, growing evidence indicates the importance of cryptic refugia for northern colonization dynamics. An important geographic region to assess this is Fennoscandia, where recolonization at the end of the last glaciation was restricted to specific routes and temporal windows. We used genomic data to analyse genetic diversity and colonization history of the bank vole (Myodes glareolus) throughout Europe (>800 samples) with Fennoscandia as the northern apex. We inferred that bank voles colonized Fennoscandia multiple times by two different routes; with three separate colonizations via a southern land-bridge route deriving from a "Carpathian" glacial refugium and one via a north-eastern route from an "Eastern" glacial refugium near the Ural Mountains. Clustering of genomewide SNPs revealed high diversity in Fennoscandia, with eight genomic clusters: three of Carpathian origin and five Eastern. Time estimates revealed that the first of the Carpathian colonizations occurred before the Younger Dryas (YD), meaning that the first colonists survived the YD in Fennoscandia. Results also indicated that introgression between bank and northern red-backed voles (Myodes rutilus) took place in Fennoscandia just after end-glacial colonization. Therefore, multiple colonizations from the same and different cryptic refugia, temporal and spatial separations and interspecific introgression have shaped bank vole genetic variability in Fennoscandia.Together, these processes drive high genetic diversity at the apex of the northern expansion in this emerging model species. K E Y W O R D S approximate Bayesian computation, Clethrionomys glareolus, climate change, cryptic refugia, genotyping-by-sequencing, population genomics | 1731 MARKOVÁ et Al.

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The legacy of climatic changes during the Pleistocene glaciations allows inferences to be made about the patterns and processes associated with range expansion/colonization, including evolutionary adaptation. With the increasing availability of population genomic data, we have the opportunity to examine these questions in detail and in a variety of non-traditional model species. As an exemplar, here we review more than two decades of work by our group and others that illustrate the potential of a single “non-model model” mammal species - the bank vole (Clethrionomys glareolus), which is particularly well suited to illustrate the complexities that may be associated with range expansion and the power of genomics (and other datasets) to uncover them. We first summarize early phylogeographic work using mitochondrial DNA and then describe new phylogeographic insights gained from population genomic analysis of genome-wide SNP data to highlight the bank vole as one of the most compelling examples of a forest mammal, that survived in cryptic extra-Mediterranean (“northern”) glacial refugia in Europe, and as one of the species in which substantial replacement and mixing of lineages originating from different refugia occurred during end-glacial colonization. Our studies of bank vole hemoglobin structure and function, as well as our recent ecological niche modeling study examining differences among bank vole lineages, led us to develop the idea of “adaptive phylogeography.” This is what we call the study of the role of adaptive differences among populations in shaping phylogeographic patterns. Adaptive phylogeography provides a link between past population history and adaptation that can ultimately help predict the potential of future species responses to climate change. Because the bank vole is part of a community of organisms whose range has repeatedly contracted and then expanded in the past, what we learn from the bank vole will be useful for our understanding of a broad range of species.