Caudata macrogenetics: Species range size predicts intraspecific genetic variation in global salamanders

Aim: Genetic diversity contains information about adaptive potential and historical demography. Intraspecific genetic variation is shaped by species natural history traits and by characteristics of geography and climate within their ranges. Salamanders (Order Caudata) are amphibians of ecological and conservation interest because of their global distribution, trait diversity, and roles within ecological communities. Here, we studied genetic variation within global salamanders to investigate the most important predictors of genetic structure and genetic diversity in this group. Location: Global. Time Period: Present. Major Taxa Studied: Salamanders. Methods: We repurposed mitochondrial DNA sequences, natural history traits, and geographic and environmental data from open-access databases for 214 salamander species. We calculated nucleotide diversity (pi) and tested for isolation by distance (IBD) and isolation by environment (IBE) within each species. We analyzed these three response variables with random forest analyses and phylogenetic comparative methods using 27 predictors expected to be associated with genetic variation (e.g., number of offspring, total range size). Results: We recovered 8,175 Cytb sequences with their associated geographic coordinates, of which 7,007 sequences were manually curated by us. Species range size was an important predictor of genetic variation (pi, IBD, and IBE). We found higher diversity in regions including the Neotropics and central-eastern Europe. The absence of strong phylogenetic signal in pi, IBD, and IBE suggests that genetic variation is shaped by local ecological and geographical factors rather than by shared ancestry. Main Conclusions: Our finding that range size was an important predictor of genetic variation aligns with theoretical expectations that species with larger ranges tend to harbor more genetic diversity. Our results underscore the importance of integrating spatial data (i.e., geographic coordinates associated with DNA sequences) into macrogenetic studies, providing valuable information for future studies and conservation strategies targeting regions with high or low genetic diversity. Keywords: Amphibians, isolation-by-distance, isolation-by-environment, mitochondrial DNA, nucleotide diversity.