The Long-tailed field mouse (Apodemus sylvaticus (L.)) has undergone a marked degree of racial differentiation on the islands of the north-west Atlantic (Iceland, Shetland, and the Hebrides). The differences have arisen as a result of the colonization of these islands after the Pleistocene by small numbers of animals carrying alleles in different proportions to those in the parental population. In contrast, the populations on some islands to the south of Britain (Jersey and Guernsey in the Channel Isles, and St. Mary's in the Isles of Scilly) are similar to A. sylvaticus from the mainland of Britain, and are likely to represent the descendants of mice which survived the Ice Ages (A. sylvaticus populations on the smaller islands–Alderney, Sark, Herm and Tresco–differ markedly from their closest relatives, and probably represent the results of recolonization following extinction in the same way as on the glaciated islands to the north).
On the mainland of Britain there is a fairly clear distinction between two groups: western and central populations, and eastern ones (which have closer affinities to French mice than western British ones). It is suggested that the two mainland British “races” may have diverged in Pleistocene refuges. Since no pelage or size genes are involved in the divergence, it would not be expected that they would be taxonomically distinct.
The data on which these conclusions are based derive from the incidence of 20 nonmetrical variants in the skulls of 1096 mice from 22 series.