Diffuse selection on resistance to deer herbivory in the ivyleaf morning glory, Ipomoea hederacea

Recent work defines coevolution between plants and herbivores as pairwise w hen the pattern of selection on resistance traits and the response to selec tion are both independent of the presence or absence of other herbivores. I n addition, for a pairwise response to selection, resistance to a focal her bivore must have the same genetic basis in the presence and absence of othe r herbivores. None of these conditions were satisfied for the ivy leaf morn ing glory, Ipomoea hederacea, and its insect, fungal, and mammalian natural enemies with a quantitative genetics field experiment. A significant negat ive genetic correlation exists between resistance to deer and generalist in sect herbivory that would preclude an independent response to selection. In addition, resistance loci under selection differ depending on the composit ion of the natural enemy community as indicated by genetic correlations bet ween deer resistances in the presence and absence of other natural enemies that differ substantially from 1. Finally, selection on deer resistance dep ends on the presence or absence of insects; in the presence of insects, gre ater deer resistance is favored, but in the absence of insects, deer resist ance is effectively neutral. These results indicate that the composition of the natural enemy community can alter both the pattern of selection and th e likely response to selection of resistance traits.