Competition for resources has long been considered a major driver of evolution by natural selection. Thus, the ability to gain access to resources not available to other individuals and species should be under strong selection. In the present study, we focus on the potential role of biting in a shrew (Crocidura russula) because this trait may confer two advantages: (1) a broadening of the dietary niche and (2) the provision of direct superiority in interspecific interactions. The model chosen is the greater white‐toothed shrew, which is considered as invasive in northern Europe and which is also known to displace native species of shrew in this area. Moreover, its distribution appears to constrain the distributional ranges of other species of shrew in the Maghreb. We use geometric morphometrics and a simple biomechanical model to describe shape variation and to evaluate the mechanical potential of the mandible of ten species of white‐toothed shrews, with a special emphasis on C. russula and Crocidura suaveolens. We find that C. russula possesses an intermediate mechanical potential linked with an intermediate level of shape variability. Our results suggest that the higher mechanical potential may explain the observed pattern of colonization of the Atlantic islands by C. russula at the expense of C. suaveolens. Finally, our results also suggest that the ability to bite hard may be under strong selection in shrews. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114, 795–807.

Many studies have shown that speciation can be facilitated when a trait under divergent selection also causes assortative mating. In Müllerian mimetic butterflies, a change in wing colour pattern can cause reproductive isolation. However, colour pattern divergence does not always lead to reproductive isolation. Understanding how divergent selection affects speciation requires identifying the mechanisms that promote mate preference and/or choosiness. This study addresses whether shifts in wing colour pattern drives mate preference and reproductive isolation in the tropical butterfly genus Melinaea (Nymphalidae: Ithomiini), and focuses on five taxa that form a speciation continuum, from subspecies to fully recognized species. Using genetic markers, wing colour pattern quantification, male pheromone characterization and behavioural assays of mating preference, we characterize the extent of genetic and phenotypic differentiation between taxa and compare it to the level of reproductive isolation. We show strong premating isolation between the closely related species M. satevis and M. marsaeus, in addition to genetic and phenotypic (colour pattern and pheromones) differentiation. By contrast, M. menophilus and M. marsaeus consist of pairs of subspecies that differ for colour pattern but that cannot be differentiated genetically. Pheromonal differentiation of subspecies was significant only for M. marsaeus, although most individuals were indistinguishable. Melinaea menophilus and M. marsaeus also differ in the strength of assortative mating, suggesting that mate preference has evolved only in M. marsaeus, consistent with selection against maladaptive offspring, as subspecific ‘hybrids’ of M. marsaeus have intermediate, non‐mimetic colour patterns, unlike those of M. menophilus which display either parental phenotypes. We conclude that a shift in colour pattern per se is not sufficient for reproductive isolation, but rather, the evolution of assortative mating may be caused by selection against maladaptive intermediate phenotypes. This study suggests that mate preference and assortative mating evolve when adaptive, and that even in the early stages of divergence, reproductive isolation can be nearly complete due to mating preferences.