Glacial cycles drive rapid divergence of cryptic field vole species

Understanding the factors that contribute to the generation of
reproductively isolated forms is a fundamental goal of evolutionary
biology. Cryptic species are an especially interesting challenge to study
in this context since they lack obvious morphological differentiation that
provides clues to adaptive divergence that may drive reproductive
isolation. Geographical isolation in refugial areas during glacial cycling
is known to be important for generating genetically divergent populations,
but its role in the origination of new species is still not fully
understood and likely to be situation dependent. We combine analysis of
35,434 single nucleotide polymorphisms (SNPs) with environmental niche
modelling (ENM) to investigate genomic and ecological divergence in three
cryptic species formerly classified as the field vole (Microtus agrestis).
The SNPs demonstrate high genomic divergence (pairwise FST values of 0.45
- 0.72) and little evidence of gene flow among the three field vole
cryptic species, and we argue that genetic drift may have been a
particularly important mechanism for divergence in the group. The ENM
reveals three areas as potential glacial refugia for the cryptic species
and differing climatic niches, although with spatial overlap between
species pairs. This evidence underscores the role that glacial cycling has
in promoting genetic differentiation and reproductive isolation by
subdivision into disjunct distributions at glacial maxima in areas
relatively close to ice sheets. Future investigation of the intrinsic
barriers to gene flow between the field vole cryptic species is required
to fully assess the mechanisms that contribute to reproductive isolation.
In addition, the Portuguese field vole (M. rozianus) shows high inbreeding
coefficient and a restricted climatic niche, and warrants investigation
into its conservation status.