A new combination, Coursetia greenmanii (Leguminosae, Papilinioideae, tribe Robinieae), is proposed because analysis of nucleotide sequence data from the nuclear ribosomal internal transcribed spacer region resolves a clade that is phenotypically distinct and ecologically and geographically centered in lowland seasonally dry tropical vegetation of the Yucatan Peninsula region. The well supported monophyly of the ribosomal sequences sampled from Coursetia greenmanii suggests that this species has had a long history independent of its close relatives. Coalescence of intraspecific samples of ribosomal sequences from this geographically confined species is strongly suggestive of an evolutionary persistent small effective population size. This is the general case for species of Coursetia and related genera of the tribe Robinieae. Coalescence of nuclear ribosomal sequences sampled from geographically restricted species in this group further suggests that evolutionary persistence could be the result of high levels of dispersal limitation in seasonally dry Neotropical vegetation that is rich in cacti and other succulent species. This is the ecological setting to which Robinieae shows phylogenetic niche conservatism.