The genus Cercis (Fabaceae: Caesalpinioideae: Cercidae) consists of approximately ten species distributed in mesic to arid climates across the warm-temperate regions of both North America and Eurasia. Variation in the shape, thickness, and upper surface of the leaves of Cercis species is correlated with climate type, and several biogeographers have invoked this pattern to support or refute hypotheses bearing on the evolution of arid biomes across the Northern Hemisphere. To employ Cercis in general biogeographical models, however, presupposes that a rigorous phylogenetic estimate of the genus exists, which has not been the case. We estimated the phylogeny of Cercis with DNA sequences of the nuclear ribosomal ITS region and a portion of the chloroplast gene ndhF. The phylogenetic relationships inferred from each region accord with one another and with results from the analysis of the combined data. The combined analysis recovered a topology in which a well supported clade of North American and western Eurasian species is nested within a paraphyletic group of Chinese species. Cercis canadensis from eastern North America is more closely related to C. siliquastrum from western Eurasia than to C. occidentalis from western North America. From DIVA and character optimization analyses, we inferred that the initial intercontinental divergence event in Cercis involved mesophytic ancestors. Subsequent inferred intercontinental divergence events involving xerophytic ancestors imply a Tertiary floristic connection between the arid regions of western North America and western Eurasia, and secondary migration to mesophytic habitats in eastern North America. Calibration of branch lengths with the fossil record suggests that the North American and western Eurasian lineages diverged between 9.1 and 32 million years ago. The oldest of these values is consistent with more or less direct trans-Atlantic dispersion across a North Atlantic land bridge (>13 million years ago), whereas the youngest requires an explanation involving long distance dispersal.

Key words: Cercis, DIVA, historical biogeography, ITS, ndhF, phylogeny