Size increase without genetic divergence in the Eurasian water shrew Neomys fodiens. Neomys fodiens

When a population shows a marked morphological change it is important to know whether such population is genetically distinct or, alternatively, the novel trait corresponds to an adaptation that might be of high ecological interest. Here, we studied a subspecies of water shrew, Neomys fodiens niethammeri, which presents an abrupt increase in skull size in a narrow area of the north of the Iberian Peninsula in comparison with the rest of the Eurasian distribution of the species and has even been suggested to represent a different species. Skulls obtained from owl pellets collected during the last 50 years allowed us to perform a morphometric analysis at the same time that short intron fragments successfully amplified from these degraded samples made possible an extensive multilocus analysis. Interestingly, no genetic divergence was detected using neither mitochondrial nor nuclear data. However, an allele frequency analysis revealed a small degree of genetic differentiation. The absence of genetic divergence and the weak structure revealed here indicate that the large form of N. fodiens does not correspond to a valid species and rather represents an extreme case of size increase of likely adaptive value, possibly due to ecological competition, which deserves further investigation.