Apodemus sylvaticus (L.) is phenotypically and taxonomically uniform over a wide area in Europe, but it has been split into 15 races or subspecies in the Hebridean and Shetland groups of islands. It is argued that with one conceivable exception (A. sylvaticus hirtensis of St Kilda) these could not be pre-glacial survivors. Furthermore the putative relationships of the different races (as revealed by a study of population samples from 26 localities) make it highly unlikely that the island races are relicts of an early post-glacial colonization. It is much more probable that they were introduced by man, and the Vikings or their descendants seem the most likely agents.
Despite having been isolated only during historical times, many of the races are quite distinct. It is suggested that this is a consequence of chance genetical differences among the founding members of each population (irrespective of any later adaptations), and that this is a method whereby rapid changes in gene frequencies can be achieved, and also provides an example of incipient speciation.