Stepwise modification of a modular enhancer underlies adaptation in a Drosophila population.

The evolution of cis regulatory elements (enhancers) of developmentally regulated genes plays a large role in the evolution of animal morphology. However, the mutational path of enhancer evolution--the number, origin, effect, and order of mutations that alter enhancer function--has not been elucidated. Here, we localized a suite of substitutions in a modular enhancer of the ebony locus responsible for adaptive melanism in a Ugandan Drosophila population. We show that at least five mutations with varied effects arose recently from a combination of standing variation and new mutations and combined to create an allele of large phenotypic effect. We underscore how enhancers are distinct macromolecular entities, subject to fundamentally different, and generally more relaxed, functional constraints relative to protein sequences.