Character shift and habitat colonization in widespread island taxa

Abstract

Ecological studies have shown that island taxa often display novel phenotypic ranges relative to mainland congeners (i.e. character shift), but most examples come from behavioural and morphological traits in animals. In this study, I hypothesize that high levels of habitat diversity on oceanic islands would also provide opportunities for character shift in plant taxa with strong colonization ability. Habitat differentiation and phenotypic variation in two resource-use traits (leaf size and specific leaf area) were analysed in taxa showing widespread distributions in the Canary Islands and across mainland Mediterranean areas: Lavandula, Olea, Periploca and Cistus. Preliminary analysis of bioclimatic data indicated that islands show subtropical conditions that are not found among the mainland habitats occupied by these taxa, which could have favoured phenotypic differentiation between these two regions. Strong evidence of character shift, however, was only found in two cases. Phenotypic data in Lavandula suggested that evolution of growth form in the island setting was associated with the expression of novel leaf traits. Evidence of character shift was also related to ecological release, most notably in Periploca, when island populations occupy a range of ecological zones distant from typical mainland conditions (e.g. zones influenced by the humid, north-east trade winds). This study shows that widespread island taxa display convergent phenotypic responses to habitat shift, which strongly suggest an adaptive pattern and stress the deterministic nature of phenotypic variation. I further discuss how the identification of these patterns provides a promising framework for the testing of hypotheses on the evolutionary mechanisms involved in phenotypic adaptation. © 2013 The Linnean Society of London.