Fig. 1.

How do new species form in organisms that occupy habitats where there is (seemingly) little opportunity for barriers to gene flow to arise? (A) Eunicea flexuosa corals in the Caribbean occur as two genetically and morphologically distinct lineages: a shallow-water lineage and a deep-water lineage. (B) Divergent selection maintains these two lineages: colonies from one end of the depth gradient have reduced survival at the opposite end. Such immigrant inviability may be crucial in promoting reproductive isolation, especially in long-lived species.