Evolutionary mechanisms of intraspecies snake venom diversification: a systematic review of population-level genetic, regulatory, and functional evidence

Snake venoms are complex and adaptive traits that show substantial variation within species. Although intraspecies venom diversity has been documented in the literature, the evolutionary mechanisms driving population-level venom variation and the strength of evidence supporting these mechanisms have not been systematically evaluated. This review aimed to synthesize population-level studies of snake venom evolution and assess the empirical support for the mechanisms underlying intraspecies venom diversification. A systematic review was carried out and reported in accordance with PRISMA 2020 guidelines. PubMed, Web of Science, and Scopus databases were searched for population-level studies examining snake venom variation using genomic, population genetics, transcriptomic, proteomic, or functional data. Eligible studies were required to provide explicit evolutionary interpretation. Information was extracted using a predefined framework that included the species, population scope, data type, analytical approach, evolutionary mechanisms, and key findings. Studies were categorized by strength of evolutionary inference based on their data and analytical approaches, ranging from descriptive and correlational evidence to population genetic testing and functional validation. The findings were synthesized narratively due to the heterogeneity in snake lineages and study design. The reviewed literature indicates that intraspecies venom diversity arises through multiple evolutionary pathways. At the population level, the most consistently supported mechanisms are natural selection acting on toxin gene coding sequences and regulatory divergence in toxin gene expression. Structural genomic processes, including gene duplication and copy number variation, are widespread but are not frequently linked directly to functional outcomes. Functional assays validating the biological effects of venom variation provide the strongest evidence for measurable biological consequences, while integration with evolutionary and ecological evidence can strengthen adaptive interpretations. Evidence at the population level indicates that snake venom diversity within species is shaped by a combination of genetic, regulatory, and functional processes, with no single mechanism operating universally. To synthesize heterogeneous population-level evidence on intraspecies venom diversification and guide future research, we propose an integrative conceptual framework to structure evidence. Integrative population-level studies that combine population genetic or genomic analyses with functional validation are essential for advancing understanding of venom evolutionary biology.