Geometric Morphometric Tests for Phenotypic Divergence Between Chromosomal Races

Abstract

As it is in other organisms, morphological divergence is an important component of speciation and evolution in shrews. Gertrude Stein is widely quoted in statistical texts as saying ‘a difference to be a difference must make a difference’: the phenotype ‘makes a difference’ for evolving because it interacts with the external world, regardless of whether that means with other shrews, with the environment or with the climate. Because mutation and drift act to change the average phenotype of a population, two populations are expected to diverge if they are separated by geographic or biological barriers to gene flow unless stabilising selection prevents it, and if two isolated populations live in different environments, then diversifying selection is likely to promote that divergence. Patterns of morphological divergence in Sorex araneus thus provide an interesting insight into the effects of chromosomal variation on evolution, especially when morphological variation is compared at increasingly higher hierarchical levels: between environments in local populations, between populations within a chromosomal race, between closely related races, between major groups of races and between species within the S. araneus group. All things being equal, morphological divergence should increase at each of these levels if Robertsonian rearrangements are important barriers to gene flow and if they arose long enough ago for morphological differences to have evolved.