Chilling Out: Cooler Climates Triggered Divergence of <i>Sabal</i> (Arecaceae: Coryphoideae: Sabaleae) at the End of the Mid-Miocene Climatic Optimum

Abstract— In this study we explore the biogeographic patterns and processes underlying the diversification of Sabal, a genus of palm with a range that expands from the tropical rainforests of northern South America into the deciduous subtropical forests of the southeastern United States. We estimate divergence times among species of Sabal using two dated fossils as calibration points to inform ancestral reconstructions of the historical distribution of Sabal , and then use floral volatile composition data in select species of Sabal to integrate plant ecological interactions into our discussion of species distributions. Our results suggest extant Sabal originated within the last 21 million years with a divergence event ∼14 million years ago that effectively split the most recent common ancestor of all southeastern United States and West Indian species from the ancestor of a clade of species with their current distribution in Mexico. This divergence event corresponds with the end of the mid-Miocene climatic optimum, which resulted in cooler climates across southeastern North America where extant cold-tolerant taxa S. minor and S. palmetto are currently distributed. Floral volatile data indicate a generalist pollination strategy involving day foraging insects. Among sampled species, floral scent profiles can mostly be characterized by quantitative differences in five main compounds: benzaldehyde, benzyl alcohol, 2-phenylethanol, ( E )-β-ocimene, and hexanal. These profiles align with major clades recovered in our phylogenetic analysis of Sabal and are indicative of evolutionary trends in pollination ecology and reproductive isolation within the genus.