Spatial Patterns of Intraspecific Genetic Diversity Follow no General Rule Across Climatic and Geographic Gradients.

Intraspecific genetic diversity (ISD) underpins key eco-evolutionary processes, yet its spatial distribution across species' ranges remains poorly understood at broad scales. Combining mitochondrial sequence alignments, species distribution models and comparative analyses, we tested whether populations closer to current niche optima exhibit higher ISD-a prediction derived from the central-marginal paradigm. With this aim, we investigated how ISD varies in relation to both climatic and geographic centroids using data from 436 herptile species (248 reptiles and 188 amphibians) from six regions across the world. We adopted a meta-analysis approach based on publicly available data. For species presenting at least five georeferenced DNA sequences from unique locations (~2.5-km resolution) within their buffered geographic range, we generated spatially explicit ISD surfaces by interpolating gene-specific data. We then quantified the relationship between ISD and the distance to both climatic and geographic centroids using species-specific Spearman's ρ coefficients. To further explore the drivers of these patterns, we applied a Random Forest framework to predict Spearman's ρ as a function of climate, ecology, geography, morphology and demography. Contrary to prevailing assumptions, the strength and direction of ISD-centroid correlations proved highly variable and species-specific, and the models consistently showed poor predictive performance. These results suggest that no uniform macroecological or evolutionary processes govern ISD patterns across taxa, but rather that ISD reflects lineage-specific histories, ecological contexts and demographic contingencies. Our findings underscore the challenges of predicting genetic diversity patterns and highlight the relevance of species-tailored approaches in conservation planning.