Modern and preserved coral reefs on islands exhibit a broad range of forms, from actively accreting fringing and barrier reefs to terraces preserved by drowning or subaerial exposure. Darwin’s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides, from fringing reef to lagoon-bounding barrier reef to atoll. Compiled data from modern systems show, however, that many islands do not follow this sequence, implying that reefs are shaped by more than island subsidence alone. We show that the diversity of modern reef morphology arises from the combined effects of island subsidence, coral growth, and glacial sea-level cycles. A model for the evolution of a reef elevation profile over the past 400 k.y. reveals that different combinations of reef accretion rate and island vertical motion produce a variety of forms that matches the observed distribution of modern reefs. This match occurs only if the model is driven by Pleistocene sea-level oscillations—few modern environments have the right combination of conditions to produce the Darwinian atoll progression.