Functional and historical drivers of leaf shape evolution in Palms (Arecaceae)

Abstract

Leaves display a remarkable variety of shapes, each potentially relevant to the function of these organs and with potential ecological advantages in certain climates. Studies correlating leaf shape with either climate or allometric constraints (how size and shape relate) focus on intraspecific variation or model organisms, and the drivers of leaf shape at a macroecological scale remain poorly known. Here, we determine associations between climatic and allometric factors with the evolution of leaf shape in a lineage with one of the highest diversities of leaf shape known, the palms (family Arecaceae). We apply a Bayesian phylogenetic mixed model to test for associations with leaf shapes (entire, pinnate, palmate, and costapalmate). We further reconstruct the ancestral leaf shape in palms using multistate speciation and extinction models. We find that high mean annual temperature correlates with dissected shapes. The ancestral leaf shape is entire, and pinnate lineages originated multiple independent times after transitions into polymorphism (species with entire or pinnate individuals). Finally, we compared the frequency of shapes with global temperatures through time through the Cenozoic. Although mean annual temperature has a positive effect on leaf dissection, the relative frequency of pinnate lineages increased as global temperatures cooled through time, potentially through colonization of open more seasonal habitats. Dissection could therefore be beneficial at high temperatures, where other leaf shaped lineages would be highly vulnerable under increased global warming. This study contributes to the understanding of leaf shape evolution and its drivers.