Ecologists have proposed that when interspecific competition is reduced, competition within a species becomes a potent evolutionary force leading to rapid diversification(1). This view reflects the observation that populations invading species-poor communities frequently evolve broader niches(2). Niche expansion can be associated with an increase in phenotypic variance(3,4) (known as character release(5)), with the evolution of polymorphisms(6-9), or with divergence into many species using distinct resources(10-13) (adaptive radiation). The relationship between intraspecific competition and diversification is known from theory(14,15), and has been used as the foundation for some models of speciation(16-20). However, there has been little empirical proof that niches evolve in response to intraspecific competition. To test this hypothesis, I introduced cadmium-intolerant Drosophila melanogaster populations to environments containing both cadmium-free and cadmium-laced resources. Here I show that populations experiencing high competition adapted to cadmium more rapidly than low competition populations. This provides experimental confirmation that competition in a population can drive niche expansion onto new resources for which competition is less severe.