Caudata macrogenetics: Geographic attributes and lineage age predict global patterns of mitochondrial genetic variation in salamanders

Aim: Genetic diversity contains valuable information about ecological and evolutionary aspects of species, such as 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 salamanders to investigate predictors of nucleotide diversity and spatial patterns of genetic differentiation. 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 and sample size (the number of sequences) were important predictors of genetic variation. 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., DNA sequences and their associated coordinates) 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, nucleotide diversity, species range size.