Timing the eastern Asian-Eastern North American floristic disjunction: Molecular clock corroborates paleontological estimates

Sequence data of the chloroplast gene rbcL were used to estimate the time o f the well-known eastern Asian-eastern North American floristic disjunction . Sequence divergence of rbcL was examined for 22 species of 11 genera (Cam psis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella , Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse a rray of flowering plants occurring disjunctly in eastern Asia and eastern N orth America. Divergence times of putative disjunct species pairs were esti mated hom synonymous substitutions, using rbcL molecular clocks calibrated for Cornus, Relative rate tests mere performed to assess rate constancy of rbcL evolution among Lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 +/- 4.30 mi llion years to recent (<0.31 million years), with most within the last 10 m illion years tin the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This es timate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian-eastern North America n floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the North ern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribu tion of taxa with low divergence, such as Menispermum. (C) 2000 Academic Pr ess.