Trait evolution and adaptive radiation in Crambe L. (Brassicaceae)

Abstract

Abstract Species richness, form, and function vary greatly throughout the tree of life, leading many to link characteristics with diversification. Which traits affect diversification and whether this pattern is different between continents and islands remain largely unknown. Using the angiosperm genus Crambe, the differences in traits (both abiotic and biotic) between the island and mainland species, and their possible effects on diversification were explored. We generate new phylogenies, based on 353 markers, which reveal the current taxonomic classifications, based on morphology, at both the species and subspecies levels are possibly inaccurate. Subspecies within this genus were found not to be closely related in the phylogeny, and the previously debated synonyms Crambe gigantea and Crambe santosii do not seem to be sister species. Furthermore, our time-calibrated phylogeny and diversification modelling suggest the evolution of Crambe on the Canary island is possibly an adaptive radiation and it is likely there was a key innovation that influenced this clade to radiate. Ancestral state reconstructions were performed for multiple abiotic and biotic traits, showing differences between the island and the mainland species. The key innovation could be the availability of new habitat (the island) itself or the abiotic factors of this habitat in particular continentality (difference between the warmest and coldest month), and UV. However, there are also indications that it could be the plant characteristics, upper segment length, or maximum plant height. Lastly, the island species occur in more arid conditions hinting towards a key innovation that made it possible for the species to survive there.