On morphological clocks and paleophylogeography: towards a timescale for Sorex hybrid zones.

Phylogeography--the study of within-species phylogenetic and geographic divergence--has been primarily the domain of molecular evolutionists because molecular markers record population structure on smaller scales than do traditional morphological traits. But when geometric morphometrics are combined with distance-based phylogenetics molar shape divergence appears to record population-level phylogeny, a fact that allows extant and fossil populations to be combined in a single phylogeographic study. The European Sorex araneus complex--a genetically complicated group composed of multiple karyotypic races and species--illustrates the principle. The phylogeographic patterns revealed by molar shape broadly agree with scenarios based on molecular data and circumstantial evidence. Importantly, the inclusion of fossil samples of known age allows minimum divergence times to be inferred. Some races of S. araneus may have diverged more than 120,000 years ago, but others may have diverged less than 14,000. Supporting evidence that molar shape can be used to reconstruct phylogeographic relationships comes from strong correlations between molar shape distances and both phylogenetic divergence time and cytochrome b sequence divergence in datasets where these variables are known independently (fossil carnivorans from a well-constrained stratigraphic setting and shrew species of the genus Sorex, respectively). However, molar shape may have a 'saturation point' beyond which it is not applicable.