A case study of coevolution:
However, in a few isolated places, there are no red squirrels, and crossbills are the most important seed predator for lodgepoles. Again, the trees are not defenseless: crossbills have more difficulty getting seeds from cones with large, thick scales. But the birds have a mode of counterattack: crossbills with deeper, shorter, less curved bills are better able to extract seeds from tough cones.
The stage is set, but the question remains: has coevolution happened? In order to show coevolution, we need evidence that suggests that the prey (the trees) have evolved in response to the predator (squirrels or birds) and that the predator has evolved in response to the prey. Researchers Craig Benkman, William Holimon, and Julie Smith set out to see if their observations would support the hypothesis of coevolution.
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Photo of lodgepole pines courtesy of Ed Jensen, Oregon State University; Photo of red squirrel courtesy of Kevin MacAskill and the Nelson Naturalist's website; Photo of Red Crossbill courtesy of Dennis Oehmke and the Illinois Raptor Center.
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California