Journal of Biogeography, 27, 391–402 © 2000 Blackwell Science Ltd Blackwell Science, Ltd Biogeographical implications of mitochondrial DNA variation in the Bockadam snake (Cerberus rynchops, Serpentes: Homalopsinae) in Southeast Asia Daryl R. Karns 1,2 , Ashley O’Bannon 3 , Harold K. Voris 1 and Lee A. Weigt 1 1 Department of Zoology, Field Museum of Natural History, 1400 S. Lake Shore Dr, Chicago, IL 60605–2496, U.S.A., 2 Department of Biology, Hanover College, Hanover, IN 47243, U.S.A. and 3 Health Sciences Center, University of Louisville, School of Medicine, Louisville, KY 40292, U.S.A. Abstract Aim The biogeography of Southeast Asia has been greatly affected by Pleistocene sea- level changes and the consequent alteration of coastline and land mass configurations. We investigated the effect of these Pleistocene events on genetic divergence in Cerberus rynchops, the Bockdam, an Asian water snake (Homalopsinae) associated with tidal mudflats and coastal mangrove forests in Southeast Asia. Location Localities for the Cerberus included the Andaman sea coast of Thailand, Sumatra, and Borneo (Sunda Shelf localities), the Philippines and Sulawesi, and from northern Australia (Sahul Shelf). Methods We analysed mtDNA sequences (12 s, 16 s, and cyt b) from seven C. rynchops populations (twenty-six specimens), from two specimens of Cerberus microlepis (a fresh- water species known only from Lake Buhi in the Philippines), and from one Enhydris enhyris (Schneider), another homalopsine used as an outgroup. Results We found a strong correlation between genetic divergence and geographical distance (r = 0.922, P < 0.001) and the biogeographic history of the region. Cerberus rynchops populations from the Sunda Shelf localities, the Philippines, Sulawesi and C. microlepis from the Philippines were genetically similar (mean divergence = 2.1%, range = 0.7– 2.8%) compared to the C. rynchops population from northern Australia (mean divergence from all other Cerberus populations = 6.6%, range = 6.0 – 7.3%). This divergence was comparable to that observed between the E. enhydris outgroup and all Cerberus populations (mean distance = 7.3%, range = 6.8 – 8.6%). Main conclusions These findings suggest a relatively high degree of movement and gene flow among Sunda Shelf localities, the Philippines, and Sulawesi (biogeographic region west of Weber’s Line) and isolation of the northern Australian (Sunda Shelf) Cerberus. Taxa will be differentially affected by these Pleistocene sea level changes dependent on their physiology and ecology. We discuss how the dispersal of the coastal, saltwater tolerant C. rynchops would have been affected by changing configurations of Pleistocene coastlines and the implications of these results for the systematics of Cerberus. Keywords Asian water snake, Homalopsinae, Pleistocene, Southeast Asia, mtDNA.