Diversity and evolution of flowering plants of the Caatinga Domain

Abstract

The Caatinga Domain (CD) in northeastern Brazil harbors the largest and most continuous expanse of the seasonally dry tropical forest and woodland biome (SDTFW) in the New World. Phytogeographical data collected over the past 10 years support previous hypotheses that recognized two major biotas in Caatinga SDTFW: the Crystalline Caatinga, mostly associated with medium to highly fertile soils in the wide Sertaneja Depression; and the Sedimentary Caatinga, mostly associated with poor sandy soils derived from patchy sedimentary surfaces. A third floristic set is represented by tall Caatinga forests. The CD is the richest SDTFW area in the New World, with 3150 species in 930 genera and 152 families of flowering plants. About 23% of the species and 31 genera are endemic to the CD. We performed phylogenetic meta-analyses to estimate times of divergence and ancestral areas for SDTFW lineages, which indicated that plant diversity in the Caatinga arose mostly by in situ speciation following Mid to Late Miocene vicariance events with two major SDTFW nuclei: (1) the northwestern Caribbean dry coast of Colombia and Venezuela; and (2) the southwestern South American dry forests of southern Bolivia and northwestern Argentina. Phylogenetic analyses also uncovered unexpected patterns of recent radiations, with evolutionarily new species and incomplete lineage sorting that sharply contrast with the most common phylogenetic patterns found in SDTFW clades. Recent, mostly Pleistocene, ecological speciation better explains the emergence of distinct biotas on sandy and karstic surfaces.