We get by with a little help from our friends: shared adaptive variation provides a bridge to novel ecological specialists during adaptive radiation.

Adaptive radiations involve astounding bursts of phenotypic, ecological and species diversity. However, the microevolutionary processes that underlie the origins of these bursts are still poorly understood. We report the discovery of an intermediate sp. 'wide-mouth' scale-eating ecomorph in a sympatric radiation of pupfishes, illuminating the transition from a widespread algae-eating generalist to a novel microendemic scale-eating specialist. We first show that this ecomorph occurs in sympatry with generalist and scale-eating specialist on San Salvador Island, Bahamas, but is genetically differentiated, morphologically distinct and often consumes scales. We then compared the timing of selective sweeps on shared and unique adaptive variants in trophic specialists to characterize their adaptive walk. Shared adaptive regions swept first in both the specialist and the intermediate 'wide-mouth' ecomorph, followed by unique sweeps of introgressed variation in 'wide-mouth' and de novo variation in . The two scale-eating populations additionally shared 9% of their hard selective sweeps with the molluscivore , despite no single common ancestor among specialists. Our work provides a new microevolutionary framework for investigating how major ecological transitions occur and illustrates how both shared and unique genetic variation can provide a bridge for multiple species to access novel ecological niches.