Environmental Stability Drives and Maintains the Global Patterns of Phylogenetic Endemism in the Marine Realm

ABSTRACT Aim Endemism is a critical aspect of biological diversity, yet the mechanisms driving it remain unclear. The leading hypothesis posits that prolonged environmental stability fosters speciation and reduces extinction rates, promoting endemism. While terrestrial studies exist, marine environments remain underexplored. This study analysed the global distribution and drivers of phylogenetic endemism in two marine taxa: Calanidae copepods and Blenniidae fish. Location Global. Time Period From the Last Glacial Maximum (LGM, past 24,000 years) to the current period. Taxa Calanidae copepods and Blenniidae fish. Methods We assessed taxonomic and phylogenetic diversity, endemism and areas of paleo‐ and neo‐endemism using Categorical Analysis of Neo‐ and Paleo‐Endemism and Piecewise Structural Equation Modelling to evaluate the influence of present‐day and historical environmental variables. Results Results showed consistent diversity patterns and significant endemism hotspots, with Calanidae in the Southern Hemisphere and Blenniidae in the Northern Hemisphere. Tropical regions, particularly the Indonesian‐Australian Archipelago (IAA), exhibited high species richness and endemism for both groups. Paleo‐endemism was observed in the Pacific for Calanidae and the IAA for Blenniidae, suggesting that these areas are either relict diversity centres or diversification hubs. Main Conclusions The Mediterranean and Caribbean Seas emerged as biodiversity cradles for Calanidae and Blenniidae respectively. Phylogenetic endemism is driven by both historical and contemporary factors, with centres of endemism emerging in regions where past environmental fluctuations have been moderate (hypothesis 1) and where cold, productive ocean currents foster the development of endemic areas (hypothesis 2). These findings underscore the importance of historical stability in ocean temperatures in shaping phylogenetic endemism, indicating that long‐term environmental stability sustains marine endemism.