Incorporating New Datatypes to Enhance Species Delimitation: A Case Study in Rice Paddy Snakes (Homalopsidae: Hypsiscopus)

Homalopsids (Old World Mud Snakes) include 59 semiaquatic species in Asia and Australasia that display an array of morphological adaptations, behaviors, and microhabitat preferences. These attributes make homalopsids an ideal model system for broader questions in evolutionary biology, but the diversity of this understudied group of snakes is still being described. Recognized species diversity in rice paddy snakes (Hypsiscopus) has recently doubled after nearly 200 years of taxonomic stability. However, the evolutionary distinctiveness of some populations remains in question. In this study, we compare mainland Southeast Asian populations of Hypsiscopus east and west of the Red River Basin in Vietnam, a known biogeographic barrier in Asia, using an iterative approach with molecular phylogenetic reconstruction, machine-learning morphological quantitative statistics, and ecological niche modeling. Our analyses show that populations west of the Red River Basin represent an independent evolutionary lineage that is distinct in genetics, morphospace, and habitat suitability, and so warrants species recognition. The holotype of H. wettsteini, a species originally described in error from Costa Rica, grouped morphometrically with the population at the Red River Basin and eastward, and those west of the Red River Basin are referred to the recently described H. murphyi. The two species may have diversified due to a variety of geological and environmental factors, and their recognition exemplifies the importance of multifaceted approaches in taxonomy for downstream biogeographic studies on speciation scenarios.