The evolution of floral displays is generally believed to be molded primarily by selection imposed by pollinators. Recently, however, it has been suggested that the evolution of floral color may also be influenced by herbivores and pathogens because several enzymes of the biosynthetic pathway that produces anthocyanin pigments are also involved in the production of various flavonoids, which can confer resistance to natural enemies. We present evidence for this type of pleiotropy in naturally occuring floral color variants of the common morning glory, Ipomoea purpurea. Specifically, we examined susceptibility to insect herbivores and fungal pathogens of genotypes with functional or non-functional copies of chalcone synthase, the first enzyme of the anthocyanin biosynthetic pathway. Genotypes with a non-functional CHS received approximately 25% more herbivore damage and exhibited twice the intensity of infection by Rhizoctonia solani. Plant fitness, as measured by seed production, was negatively correlated with amount of herbivore damage. These results indicate that the equilibrium frequences of the CHS alleles in natural populations may be governed by diffuse selection imposed by herbivores and pathogens, as well as by pollinators.

Key words: anthocyanins, coevolution, diffuse selection, resistance