Post-glacial partitioning of mitochondrial genetic variation in the field vole

Genetic markers are often used to examine population history. There is considerable debate about the behaviour of molecular clock rates around the population-species transition. Nevertheless, appropriate calibration is critical to any inference regarding the absolute timing and scale of demographic changes. Here, we use a mitochondrial cytochrome b gene genealogy, based entirely on modern sequences and calibrated from recent geophysical events, to date the post-glacial expansion of the Eurasian field vole (Microtus agrestis), a widespread temperate mammal species. The phylogeographic structure reflects the subsequent expansion of populations that went through bottlenecks at the time of the Younger Dryas (ca 12 000 years BP) rather than the Last Glacial Maximum (LGM, ca 24 000 years BP), which is usually seen as the time when present-day patterns were determined. The nucleotide substitution rate that was estimated here, ca 4 x 10(-7) substitutions/site/year, remains extremely high throughout the relevant time frame. Calibration with similarly high population-based substitution rates, rather than long-term rates derived from species divergence times, will show that post-LGM climatic events generated current phylogeographic structure in many other organisms from temperate latitudes.