Gene flow and morphological divergence were measured among 12 populations of a common species of rainforest passerine. Populations in the forest and the ecotone (the transition zone between the African rainforest and savanna) are morphologically divergent, despite high gene flow, and morphological differences between habitats are as large as those found between related species. in contrast to past theories of rainforest speciation, which emphasize geographic isolation, these results suggest that natural selection may play an important role in generating rainforest biodiversity. Because ecotone habitats may be a source of evolutionary novelty, greater attention should be paid to their conservation in order to preserve the processes that may be important to maintain rainforest diversity.