The ecological hypothesis of speciation is that reproductive isolation evolves ultimately
as a consequence of divergent natural selection on traits between environments. Ecological
speciation is general and might occur in allopatry or sympatry, involve many agents
of natural selection, and result from a combination of adaptive processes. The main
difficulty of the ecological hypothesis has been the scarcity of examples from nature,
but several potential cases have recently emerged. I review the mechanisms that give
rise to new species by divergent selection, compare ecological speciation with its
alternatives, summarize recent tests in nature, and highlight areas requiring research.