A synthesis of work on the evolutionary origins of biodiversity, on the impacts of biodiversity on ecosystem functioning and on mechanisms of multi-species coexistence suggests a simple unifying hypothesis: that the same interspecific tradeoffs that lead to speciation also cause species to coexist, and cause a strong dependence of ecosystem functioning on biodiversity. This “Universal Tradeoff Hypothesis” suggests a surprisingly tight linkage among evolutionary, population and ecosystem processes. It also suggests that the loss of biodiversity, whether from species extinctions, community simplification, or loss of genetic variation within populations, can have serious implications for global environmental sustainability.