An Original Biogeographic Framework for the Origin and Diversification of Freshwater Fish Biodiversity in the Niger-Benue System, West Africa

This dissertation develops an original biogeographic framework for understanding the evolutionary origin and diversification of freshwater fish biodiversity in the Niger-Benue river system, integrating molecular phylogenetics, historical biogeography, and macroecological analysis across Nigerian and broader West African fish assemblages. The Niger-Benue confluence is one of the most species-rich freshwater zones in Africa, yet no synthetic biogeographic theory for the origin of this diversity exists, and the relative contributions of vicariance, dispersal, and adaptive radiation to current species richness have not been evaluated. The dissertation assembles a novel multi-locus molecular dataset for 280 fish species representing all major orders and families of the Niger-Benue system, using museum specimens and freshly collected material from the North Central, South East, and South South zones. Bayesian and likelihood-based phylogenetic methods with fossil and palaeogeographic calibrations are used to reconstruct time-calibrated phylogenies for each order. Ancestral area reconstruction using BioGeoBEARS evaluates alternative biogeographic models. A macroecological analysis relates diversification rates to palaeoclimatic and geomorphological covariates. The dissertation proposes the Niger-Benue Diversification Mosaic Theory (NBDMT) as an original contribution, identifying three diversification episodes: a Miocene vicariance event associated with African topographic uplift, a Pliocene dispersal radiation following river capture events, and a Pleistocene climate-driven speciation pulse in forest-stream specialists. The NBDMT provides a predictive framework for identifying speciation hotspots applicable to future phylogeographic studies. Keywords: biogeography, freshwater fish, Niger-Benue system, diversification, phylogenetics.