Seen from space, the Amazon basin is a green mat interrupted only by the branches of a massive river system (see the figure). Since the Amazonian expeditions of Alfred Russel Wallace in the 1850s, bio-geographers have sought to clarify the forces driving the remarkable morphological and genetic diversity of terrestrial species in the Amazon basin, the world's richest biome. Several million years of evolution have produced a mosaic of overlapping morphologically and genetically distinct terrestrial species, resulting in areas of endemism. Much research has investigated the diversity of Amazonian terrestrial species, whereas freshwater species restricted to the Amazon and its many tributaries have been little studied. This oversight is now remedied by Fernandes et al. , who report on page 1960 of this issue the results of their comprehensive survey of electric fish species across a 2000-km transect of the Amazon basin.
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