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 ‘wide-mouth’ scale-eating ecomorph in a sympatric radiation of Cyprinodon 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 C. variegatus and scale-eating specialist C. desquamator 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 desquamator and the intermediate ‘wide-mouth’ ecomorph, followed by unique sweeps of introgres...