Evolutionary radiations of Proteaceae are triggered by the interaction between traits and climates in open habitats

Aim: Ecologically driven diversification can create spectacular diversity in both species numbers and form. However, the prediction that the match between intrinsic (e.g. functional trait) and extrinsic (e.g. climatic niche) variables may lead to evolutionary radiation has not been critically tested. Here, we test this hypothesis in the Southern Hemisphere plant family Proteaceae, which shows a spectacular diversity in open mediterranean shrublands in the Southwest Australian Floristic Region (SWAFR) and the Cape Floristic Region (CFR). Species in the Proteaceae family occupy habitats ranging from tropical rain forests to deserts and are remarkably variable in leaf morphology.

Location: Southern Hemisphere.

Methods: We built a phylogenetic tree for 337 Proteaceae species (21% of the total), representing all main clades, climatic tolerances and morphologies, and collected leaf functional traits (leaf area, sclerophylly, leaf shape) for 261 species and climatic niche data for 1645 species. Phylogenetic generalized least squares regression and quantitative-trait evolutionary model testing were used to investigate the evolutionary pathways of traits and climatic niches, and their effect on diversification rates.

Results: We found that divergent selection may have caused lineages in open vegetation types to evolve towards trait and climatic niche optima distinct from those in closed forests. Furthermore, we show that the interaction between open habitats, dry, warm and/or mediterranean climates, and small, sclerophyllous, toothed leaves increases net diversification rates in Proteaceae.

Main conclusions: Our results suggest that the evolution of specific leaf adaptations may have allowed Proteaceae to adapt to variable climatic niches and diversify extensively in open ecosystems such as those in the CFR and SWAFR. This match between morphology and environment may therefore more generally lead to evolutionary radiation.