II. Phylogeography of Northeast Asia
Northeast Asia (Korean Peninsula, China, and Russia) has had a complex geological and climatic history, which affected the evolution and dispersal of organisms in the region. During the Pleistocene, major geological events (e.g., the opening of the Yellow Sea, forming of major mountain ranges) and dramatic climate change are believed to have affected the distribution of terrestrial vertebrates (Lee et al., 2008; Zhang et al., 2008; Sakka et al., 2010; Zhang et al., 2012; Kim et al., 2013; Koh et al., 2013; Fong et al., 2016; Borzée et al., 2017; Fong et al., 2020). In particular, studies confirmed that the Korean Peninsula was not covered by glaciers during the latest glacial cycle (Kong, 2000; Yi & Kim, 2010), and this region played an important role as a refugium in preserving genetic diversity (Lee et al., 2008; Zhang et al., 2008; Kim et al., 2013; Fong et al., 2020).
Studies on extant amphibians in this region have clarified the phylogeography of the Chinese black-spotted frog (Pelophylax nigromaculatus ) (Zhang et al., 2008), Oriental fire-bellied toad (Bombina orientalis ) (Fong et al., 2016), Japanese tree frog (Dryophytes japonica group) (Dufresnes et al., 2016), brown frog (Rana dybowskii ) (Yang et al., 2017), water toad (B. stejnegeri ) (Fong et al., 2020), and Asiatic toad (B. gargarizans ) (Zhan & Fu, 2011; Borzée et al., 2017). Each study had species-specific goals, but a common finding was that there is genetic divergence between China and Korea likely due to geology (mountain and oceanic barriers) and Pleistocene glacial cycles. For B. gargarizans , Zhan and Fu (2011) inferred western China to be a major refugium for B. gargarizans owing to high genetic diversity in the region, while Borzée et al. (2017) inferred that this species dispersed westward from the Korean Peninsula across land bridges during low sea levels. We provide more clarity on the evolution and dispersal process of B. gargarizans in Northeast Asia.