Divergent evolutionary processes associated with colonization of offshore islands

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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.

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We acknowledge receipt of a Marie Curie Intra European Fellowship (to N.M.), support from the Swiss National Science Foundation (projects 31003A?127377, 3100A0?112072 and 3100?126074) to L.E. and G.H., funding from SYNTHESYS2 made available by the European Community Research Infrastructure under FP7 (‘Synthesis of Systematic Resources’, 226506?CP?CSA?Infra) to S.B., a Wellcome Trust University award to K.M.D. (GR071037) and overarching funding from the Arts and Humanities Research Council (project grant 119396).

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demographic analysis; genetic replacement; island colonization; Microtus arvalis; phylogeography


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Molecular Ecology (2013), 22(20), 5205–5220

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