Phylogenomics, biogeography and evolution in the American genus Brahea (Arecaceae)

Slow rates of molecular evolution at low taxonomic levels hamper studies of relationships among species and subsequent biogeographic and evolutionary analyses. An example is the genus Brahea, which is among the most poorly understood lineages of American palms and is characterized by a wide variety of growth forms and intermediate morphological features. We generated c. 400 kb of genome-scale data from all three genomes for the 11 currently described Brahea spp. to infer phylogenetic relationships, reconstruct ancestral growth form, estimate ancestral geographical ranges and test for niche differences among closely related species with geographical overlap. Relationships receive strong support and conform to previous subgeneric assignments, except for placement of the dwarf species B. moorei in Brahea subgenus Erythea. Our robust phylogenetic hypothesis reveals trends in growth form, including an overall increase in height in the B. armata clade and independent evolution of dwarf forms from taller ancestors in the B. pimo and B. dulcis clades. Ancestral range estimation reveals roles of dispersal (e.g. B. edulis on Guadalupe Island) and putative divergence within geographical regions in some cases (e.g. in the B. armata clade) but is equivocal in others (e.g. in the B. pimo clade). We find evidence of niche non-equivalency among species in the B. armata clade in north-western Mexico and some evidence of niche non-equivalency between B. berlandieri and B. dulcis, the former of which is synonymized under B. dulcis. Our findings have implications for the complex biogeographic history in Central America and Mexico, suggesting that divergence within regions and dispersal are the predominant processes of species diversification. Future studies should include population-level sampling across the genus, along with morphological and ecological information, to assess distinctness among species and, particularly, levels of gene flow, in an integrative fashion.