Laboratory populations of a green alga cultured in the dark with an organic substrate evolved into efficient heterotrophs with faster growth, higher fitness, and increased responsiveness to substrate concentration. Their phenotypes were almost entirely attributable to selection, rather than to history or ancestry. The fitness of evolved lines in the light was uniformly depressed, presumably through the accumulation of conditionally deleterious genes governing photosynthesis. Some evolved lines were no longer able to grow in the light and are thereby permanently isolated from their ancestors. Specialized autotrophs and heterotrophs may often evolve in algae through long-term shifts in the conditions of growth.