Genetic variability and the relationships between clones of mitochondrial DNA (mtDNA) provide data which can be used to construct hypotheses about the biogeographical and postglacial colonization history of Fennoscandian species. We have investigated fragment differences in mtDNA among 24 Wood mice (Apodemus sylvaticus) from 8 localities, an'd 9 Yellow necked mice (A. flavicollis) from four North European localities.
The greatest intraspecific sequence divergence found between mtDNA clones was 1.40% within A. sylvaticus and 1.00 % within A. flavicollis. The mean pairwise divergence in samples from a local population (n = 14) of A. sylvaticus was only 0.003 %. Interspecific fragment comparison of mtDNA from A. sylvaticus and A. flavicollis give a sequence divergence close to 10%: However, comparison of genetic distances based on mtDNA between the two Apodemus species shows that they are more distantly related than suggested by nuclear distances. The observed discrepancy between distances estimated from mitochondrial and nuclear DNA suggests that mtDNA divergence predated divergence in nuclear genes or that there was an influx of nuclear variation during the speciation process or in connection with the expansion and contractions of populations during interglacial and glacial periods.