Evolution

Pine Cones, Squirrels, and Crossbills

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Science  25 Apr 2008:
Vol. 320, Issue 5875, pp. 426-427
DOI: 10.1126/science.320.5875.426d

Selection for coevolutionary adaptations is buffeted by geographical variation in community composition and species interactions. To explore how geographic selection mosaics are influenced by resource variability, feeding specialization, and vagility of interacting taxa, Parchman and Benkman examine interactions in the western United States among ponderosa pine (Pinus ponderosa), two allopatric species of tree squirrel (Sciurus), and one type of red crossbill (Loxia curvirostra). Feeding by western gray squirrels (S. griseus) selects for cone traits (such as size) that greatly reduce crossbill use of pine seeds, and crossbill specialization on ponderosa is limited to areas outside the gray squirrel's range. Preferentially foraging on inner bark, Abert's squirrels (S. aberti) cut twigs with developing cones, thereby depressing seed supply and lowering the selective impact of crossbills on the pine. Thus, crossbill-ponderosa coevolution is strongest in the absence of both squirrels. But high interannual variation in cone crop encourages the birds to be nomadic and move regularly among areas with and without Abert's squirrels. Such movements prevent strong selection mosaics and the local differentiation of crossbill populations found where they feed on more consistent seed supplies. — ShJS

Evolution 62, 348 (2008).

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