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Central Africa's roiling, rapid Lower Congo River is one of the most biologically diverse rivers in the world. More than 320 fish species call it home, and about 90 of these live nowhere else. Ichthyologists from the American Museum of Natural History are working with geographers and hydrologists to explain how this extraordinary species richness came to be. Using the latest genetic methods and hydrological equipment, the team is exploring how regions of whitewater have acted as a barrier to fish movement over tens of thousands of years, thus allowing species to diverge. The results could overturn long-held assumptions about how fish evolve and demonstrate how high-resolution mapping can aid the study of freshwater communities worldwide.