REVIEW ARTICLE Vicariant speciation due to 1.55 Ma isolation of the Ryukyu islands, Japan, based on geological and GenBank data Soichi OSOZAWA 1 , Zhi-Hui SU 2,3 , Yuichi OBA 4 , Takashi YAGI 5,6 , Yasushi WATANABE 7 and John WAKABAYASHI 8 1 Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, Japan, 2 JT Biohistory Research Hall, Osaka, Japan, 3 Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Japan, 4 Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan, 5 Department of Biology, Graduate School of Science, Osaka Prefecture University, Sakai, Japan; 6 Department of Life Science, Dongguk University, Seoul, Korea, 7 GIS Okinawa Laboratory, Okinawa, Japan; and 8 Department of Earth and Environmental Sciences, California State University, Fresno, California, USA Abstract The Ryukyu island arc, originally a continental margin arc, separated from the Chinese continent by the rifting of the Okinawa trough, a process which began at 1.55 million years ago (Ma) and continues to the present. In addition, the Ryukyu arc was simultaneously divided into the northern Amami–Okinawa and southern Yaeyama islands by the Kerama rift valley, and consequently formed two isolated island units. The Kuroshio warm current began to flow into the Okinawa trough from the Yonaguni Strait, and flow out through the Tsushima and Tokara straits also at 1.55 Ma, and these seaways effectively acted as barriers between the Ryukyu islands and Taiwan, China and Japan. Through this geological process, vicariant speciation generated Ryukyu endemic animal species. We support this hypothesis by drawing linearized maximum likelihood (ML) phylogenetic trees of the species in four endemic insect groups (peacock butterfly, Chinese windmill butterfly, golden-ringed dragonfly, window firefly) using GenBank sequence data. We determined the precise branching ages for these phylogenetic trees, and show simultaneous speciation at 1.55 Ma for Amami–Okinawa and Yaeyama units. The Taiwan and Tsushima straits, barriers between Taiwan and China, and Japan and Korea, respectively, did not form sufficient barriers to migration during glacial low stands, and species were intermingled. A marine embayment may have posed as a migration barrier between northern and southern China in the Quaternary or a little earlier. From our study we also estimate the precise molecular evolution rate and justify the molecular clock. Key words: endemic insect species, Kerama Strait, molecular evolution rate, Okinawa trough, phylogenetic tree, Tokara Strait. INTRODUCTION Precise age determination for a geological event is an important and commonly challenging objective for geologists. The molecular clock appears to be an attrac- tive and powerful tool for evolutionary biologists, if DNA can be treated as a kind of fossil that records time and/or history. In practice, a molecular phylogenetic biologist requires geological and paleontological input to assign precise and reliable divergence ages to a phy- logenetic trees of organisms (e.g. Nei & Kumar 2000). The fossil record, however, is commonly poor and seldom has sufficient resolution to apply directly to quantitative assessment of molecular evolution rate. In this paper, we combine newly determined ages of geo- logical events with DNA analysis of endemic insect species on the Ryukyu islands to estimate branching ages and patterns for their phylogenetic trees and Correspondence: Soichi Osozawa, Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. Email: osozawa@m.tohoku.ac.jp Received 9 August 2012; accepted 7 April 2013. Entomological Science (2013) 16, 267–277 doi:10.1111/ens.12037 © 2013 The Entomological Society of Japan