Abstract

THE role of selective agents in the origin of evolutionary novelties has been controversial1−3 and has remained outside the realm of experiments. Here we experimentally determine both the benefits of a single trait and the advantages accrued during the presumed sequence of evolutionary steps leading to the fully specialized structure. By comparison of red crossbills (Loxia curvirostra, L.), in which the mandibular crossing has been removed, with controls and with the related but less specialized pine siskin (Carduelis pinus Wilson), we show the advantage of the mandibular crossing in the extraction of seeds from partially closed conifer cones. We use the natural regrowth of the mandibles to mimic the evolution of mandibular crossing from an unspecialized ancestor, and use the relationship of foraging efficiency to mandibular regrowth to determine a scheme for its (gradual) evolution.