The remarkable floristic similarities between temperate eastern Asia and eastern North America have been recognized for over a century. Despite an unusually healthy fossil record, we are far from a general biogeographic hypothesis that will prove to be robust across many distinct taxa. The most informative approach to this problem will combine estimates of genetic divergence, preferably for several different data sets, with explicit methods of cladistic and biogeographic analysis. Such an approach is described herein for the woody genus Gleditsia (Leguminosae: Caesalpinioideae), which contains two hypothesized eastern Asian-eastern North American vicariad species-pairs, in addition to other intra- and intercontinental disjunctions. The study will first use cladistic analysis of different molecular data sets to recognize putative monophyletic groups. Area cladograms will be constructed to discern intra- and intercontinental disjunctions within monophyletic groups. Tests of competing biogeographic hypotheses (especially dispersal vs. vicariance) for the origin of eastern Asian-eastern North American disjunctions (and others) will derive from the topology of area cladograms coupled with estimates of genetic divergence provided by the molecular data. Gleditsia will thus serve as a model for similar approaches in other genera, with the ultimate objective being to understand the origins and biogeographic history of other northern hemisphere Tertiary disjunctions.
