Discussion I. Genetic structure of B. gargarizans
We contribute to understanding the evolutionary history of B. gargarizans by combining broad geographic sampling with a multilocus genetic dataset in a single study. Our mtDNA (ND2 ) data analysis results support B. gargarizans being divided into two groups that are further divided into eight clades (Group I: Clades A, B, C, D, H; Group II: Clades E, F, G) (Fig. 2 ). The genetic status of seven clades (A–G) was identified in the previous study (Zhan & Fu, 2011), while an additional clade (H) was newly identified in our study (Fig. 2 ). We uncovered weak phylogeographic pattern for B. gargarizans , where the genetic differentiation did not strongly match any geographic pattern. As Zhan and Fu (2011) suggested, for such a genetic pattern to appear, the geographic distribution of B. gargarizans would have expanded rapidly after genetic differentiation occurred.
We propose two hypotheses regarding the weak phylogeographic pattern ofB. gargarizans related to their adaptations to environmental stressors. First, B. gargarizans is large-bodied and has a dry, tough skin, allowing it to survive in xeric conditions and to disperse long distances across land relative to other amphibian species. These features of B. gargarizans likely played a role in its wide distribution across China. If the range expansion involved many individuals and occurred soon after genetic differentiation, such an undifferentiated phylogeographic pattern could result.
Second, we suggest that anthropogenic effects contributed to the lack of a clear phylogeographic pattern in B. gargarizans . In China, a traditional Chinese medicine (hua chan su, 華蟾素) extracted from skin secretions of toads (including B. gargarizans ) has been used as medicine for thousands of years (Su & Nu, 2001; Meng et al., 2012; Cheng et al., 2019). A current online search for toad farms identifies locations operating in various regions in China (Table S7 ). Any escape or release of translocated individuals, followed by reproduction with native individuals would contribute to obscuring phylogeographic patterns.