The latitudinal speciation gradient in freshwater fishes: higher speciation at higher latitudes in the northern hemisphere

Speciation rates are a key driver of diversity patterns and are often used to explain the latitudinal diversity gradient (LDG). However, latitudinal variation in speciation rates at both assemblage and species levels remains poorly explored in freshwater fishes. This highlights a gap in understanding the mechanisms driving geographic biodiversity gradients in freshwater fishes. Here, we investigated the latitudinal speciation gradient in freshwater fishes, using a comprehensive database of freshwater fish distributions and phylogenetic relationships of Actinopterygian fishes. We estimated speciation rates using three metrics (BAMM, DR, and ClaDS) and evaluated the latitudinal speciation gradient through spatial and phylogenetic regressions at assemblage and species levels. Finally, we analyzed those patterns based on the species assemblage's phylogenetic diversity and structure. Our results show that areas and species with the highest speciation rates were located in the tropics. However, the general assemblage pattern revealed a positive relationship between absolute latitude and speciation rates. This relationship is generally absent in tropical regions below 23.8° and became significant only at higher latitudes, particularly in the Northern Hemisphere. We do not find a significant relationship at the species level, mainly due to the strong influence of hyper-diverse groups like Cichliformes. When Cichliformes were excluded, the relationship between latitude and speciation became significant, aligning with assemblage-level results. The areas whit higher speciation rates also match with areas whit recent radiation and higher clustering in all latitudes. However, septentrional assemblages show high speciation rates, which reflects environmental filtering and dispersal events consistent with the glaciation and deglaciation in the Pleistocene.