The fossil records show that since the Middle Pleistocene, lemmings (Dicrostonyx, Lemmus) have been sympatric across their ranges. I compared mitochondrial DNA (mtDNA) diversity estimates between the two genera to infer a difference in demographic history resulting from biotic responses to Quaternary environmental fluctuations. The mtDNA diversity estimates in Lemmus consistently exceed those in Dicrostonyx on regional and continental spatial scales. However, as opposed to the mainland, the diversity estimates in Lemmus are lower than those in Dicrostonyx on Wrangel Island. Under the assumption of equal mutation rates, a difference in diversity estimates reflects a difference in the historical effective size. On a regional scale, the low mtDNA diversity in Dicrostonyx suggests it suffered a reduction in effective population size, probably due to range contractions during warming events in the Holocene. On a continental scale, the low average divergence in Dicrostonyx indicates a dispersion event after the range contraction in Eurasia to a single refugium, also due to warming events in one of the interglacials. In contrast to Dicrostonyx, the high mtDNA, diversity in Lemmus gives no indications for a reduction in its effective size during late Quaternary warming events. This implies that the two historically codistributed genera responded differently to Quaternary environmental changes, even if their differences in biotic responses are undetectable in the Arctic fossil record. This study demonstrates that molecular genetic data increase the resolution of palaeoecological analyses at the community level.
