“Only One, Two at the Most”: George Davis' Insights into Adaptive Radiations of Freshwater Gastropods as an Early Evolutionary Systematics Approach

This review examines the early contributions of George Morgan Davis (1938–2024) to the systematics and evolution of some freshwater gastropods, with emphasis on cerithioidean taxa, e.g., Brotia H. Adams, 1866 (today considered as Pachychilidae) and Semisulcospira O. Boettger, 1886 (Semisulcospiridae). Building on taxonomic revisions and the pioneering use of electrophoretic data, Davis developed broad hypotheses on speciation (including allopatric versus sympatric origins), radiation, hybridisation, introgression, and historical biogeography that anticipated integrative approaches later formalised in evolutionary systematics. His seminal 1982 review on “adaptive radiation” also addressed fundamental cladistic questions and underscored the necessity of synthesising multiple lines of evidence to resolve species boundaries and diversification processes. His original analyses of endemic radiations of Semisulcospira in Japan, in particular Lake Biwa, and triculine gastropods in the Mekong River, in comparison to other riverine radiations, e.g., of Brotia in the Kaek River in Thailand, highlighted the interplay of extrinsic factors (tectonics, drainage history, ecological complexity) and intrinsic traits (e.g., viviparous strategies, parthenogenesis) in shaping diversification. Although developed with limited phylogenetic and biodiversity data, his multidimensional “phylogenetic topology” can be viewed as an attempt to integrate all available lines of evidence from the various analytical components to unite phylogenetic inference, historical biogeography, and ecological causation based on intrinsic characters. Reconsidered here against modern molecular phylogenetic and genomic insights, Davis' work appears strikingly prescient, foreshadowing current integrative frameworks for systematics, albeit at the same time pointing to the limitations of his early synthesis. By situating Davis' legacy within ongoing debates on species concepts and delimitation, speciation modes, and the role of historical versus ecological drivers, this study underscores how past conceptual advances continue to inform contemporary biodiversity research. Re-evaluating Davis' hypotheses further illustrates how he helped shift malacology beyond pure taxonomy and classification towards truly “evolutionary” systematics that remains central to understanding radiations, not just in freshwater gastropods.