If colonizing populations are displaced into an environment that is often very different from that of their source1, they are particularly likely to diverge evolutionarily, the more so because they are usually small and thus likely to change by genetic restructuring or drift2,3. Despite its fundamental importance, the consequence of colonization for traits of founding populations have primarily been surmised from static present-day distributions1,2,4,5, laboratory experiments6 and the outcomes of haphazard human introductions7-9, rather than from replicated field experiments. Here we report long-term results of just such an experimental study. Populations of the lizard Anolis sagrei, introduced onto small islands from a nearby source, differentiated from each other rapidly over a 10-14-year period. The more different the recipient island's vegetation from that of the source, the greater the magnitude of differentiation. Further, the direction of differentiation followed an expectation based on the evolutionary diversification of insular Anolis over its entire geographic range. In addition to providing a glimpse of adaptive dynamics in one of the most extensive generic radiations on earth, the results lend support to the general argument that environment determines the evolution of morphology.