Oceanic islands have been a test ground for evolutionary theory, but here, we focus on the possibilities for evolutionary study created by offshore islands. These can be colonized through various means and by a wide range of species, including those with low dispersal capabilities. We use morphology, modern and ancient sequences of cytochrome b (cytb) and microsatellite genotypes to examine colonization history and evolutionary change associated with occupation of the Orkney archipelago by the common vole (Microtus arvalis), a species found in continental Europe but not in Britain. Among possible colonization scenarios, our results are most consistent with human introduction at least 5100 bp (confirmed by radiocarbon dating). We used approximate Bayesian computation of population history to infer the coast of Belgium as the possible source and estimated the evolutionary timescale using a Bayesian coalescent approach. We showed substantial morphological divergence of the island populations, including a size increase presumably driven by selection and reduced microsatellite variation likely reflecting founder events and genetic drift. More surprisingly, our results suggest that a recent and widespread cytb replacement event in the continental source area purged cytb variation there, whereas the ancestral diversity is largely retained in the colonized islands as a genetic ‘ark’. The replacement event in the continental M. arvalis was probably triggered by anthropogenic causes (land-use change). Our studies illustrate that small offshore islands can act as field laboratories for studying various evolutionary processes over relatively short timescales, informing about the mainland source area as well as the island.

Phylogenetic tree from cytb sequences of common voles, Microtus arvalisTreeBase entry containing DNA sequence alignment of mitochondrial cytb gene, phylogenetic tree from the Bayesian inference analysis, details of the analysis. This constitutes Figure 2 in the paper.Microsatellite genotypes of common voles, Microtus arvalisGenotypes of 14 microsatellite loci in 24 populations of common voles in Arlequin format. Binned fragment size is given. Samples are labelled with isolate number.Microtus_arvalis_Orkney_Msat_24Pops_14Loci.arpInput file for isolation-with-migration analysis of common voles, Microtus arvalisThe file contains mitochondrial cytb sequences and microsatellite genotypes of common voles formatted for the IMa program. Samples are labelled with isolate number.Microtus_arvalis_Orkney_cytb_msat_IMa.txtMorphological measurements of common voles, Microtus arvalisMorphological coordinates collected from 2D images of common vole skulls. Raw cartesian coordinates in pixels are given in columns Rawx1, Rawy1 through to Rawx30, Rawy30. Columns with respective names starting with Ali are Proscrustes coordinates; the shape variables after the Procrustes superimposition. Samples are labelled with the population code and number from the individual voucher.Microtus_arvalis_Morpho_Data.xlsGenBank population set informationAncient DNA sequences of cytb from common voles, Microtus arvalis, are in GenBank under PopSet: 302202495. Modern complete sequences of the cytb gene from common voles, Microtus arvalis, are in GenBank under PopSet: 304634227.GenBank_population_sets.txt