The House mice (Mus musculus L.) of the Faroe Islands (62°N, 7°W) are frequently quoted as examples of rapid evolution, because they seem to be clearly differentiated both from mice in other parts of the world, and between different islands within the Faroe group despite being introduced through human agency within the last millennium. They are also of interest in being among the most climatically stressed mice in the Northern Hemisphere.

The present study is an attempt to determine the extent of differentiation between the populations on the six Faroe islands which harbour mice. It is based on size and organ weights, measurements on the mandibles and scapulae, non-metrical variation of the skeleton, and allozymic frequencies at 22 loci. Distance statistics calculated for all five groups of data between samples from each island compared with every other, showed that all the populations were clearly distinct. However the distances calculated from the different data were disconcertingly heterogeneous. The most likely explanation is that the different characteristics scored each depend on a relatively small number of different genes.

Taking all the results together, it seems most probable that mice first entered the Faroes via the main port of Torshavn and spread from there to Nols0y, Hestur and Sand0y, and from Sand0y to Fugloy and Mykines. The large inter-island differences can be attributed primarily to “instant sub-speciation” produced by each colonization depending on a probable small number of effective founders.