• adaptive radiation;
  • parasites;
  • directional natural selection;
  • population differentiation


Adaptive radiation is characterized by rapid phenotypic diversification as a result of utilizing different environments. Red crossbills (Loxia curvirostra Linnaeus – complex) have diversified in bill size and shape, and overall size, in response to differences in the conifer cones that hold the seeds they almost exclusively feed upon. However, a recent study showed how a bill of suboptimal size for foraging has evolved due to antagonistic selection by scaly leg mites Knemidokoptes jamaicensis. This suggests that the current variation in morphology within the adaptive radiation of crossbills may not be exclusively the result of adaptation to alternative resources. Using an independent set of populations, we found that the surprising and little-understood relationship between crossbill morphology and infection with mites is repeatable. Assuming mites depress survival, this relationship would result in directional, not stabilizing selection on morphology. We also find that the rates of infection can differ dramatically between populations, potentially depending on their ecologies. These findings suggest that morphological evolution within the adaptive radiation of crossbills may partly occur for reasons unrelated to resource use.