Genetic differentiation and biogeography of the masked shrew in Atlantic Canada

Abstract

The masked shrew (Sorex cinereus) occurs naturally on a number of small to large islands in Atlantic Canada, and 22 individuals were introduced into Newfoundland from New Brunswick in 1958. Genetic variation detected by protein electrophoresis of 34 presumptive gene loci was assayed in five island and three mainland populations from this region to compare rates of differentiation among the populations and to investigate genetic relationships among populations in the context of historical biogeographic events constructed from the geological literature. We found considerable differentiation among populations which can be attributed to different origins for the populations in the post-Wisconsin glacial phase, and to apparently rapid evolution in small isolated populations, especially for the small island of Bon Portage, Nova Scotia. Allele frequencies are more heterogeneous among the island populations and exhibit classic neutral behaviour in that rare allelles are absent and shifts in allele frequency occur to either side of mainland means. There is no correlation of allele frequencies with geographic proximity of populations as might be expected under selectively based clines, or, alternatively, under isolation by distance expected with a diffusive wave of colonization in the region. Instead, the genetic data are compatible with random drift of neutral alleles in populations of various efffective sizes isolated from each other by rising sea levels in the last 20 000 – 5000 years. The considerable divergence that has occurred among the island populations (FST = 0.238) may be indicative of incipient speciation or subspeciation. This subdivision does not appear to be founder-induced but is more likely the product of gradual drift in and interruption of homogenizing gene flow among relict populations that existed during the Pleistocene.