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The evolution of specialized floral phenotypes in a generalist pollination environment.
AIGNER, PAUL*,1, 1 Department of Biology, Riverside, CA
ABSTRACT- The evolution of specialized floral phenotypes (i.e., adaptations to particular pollinator types) often occurs in a fine-grained environment in which plants initially receive services from more than one pollinator type. Optimality models predict that the evolutionary outcome in such an environment depends on the exact form of a fitness trade-off in adaptation to individual pollinator types. Trade-offs may be absent, symmetric, or asymmetric, but only asymmetric trade-offs result in natural selection for specialized phenotypes. The genus Dudleya (Crassulaceae) consists mostly of species that are phenotypic and ecological generalists for pollination. Flowers are five-parted, radially symmetric, and architecturally uncomplicated. They are visited by a variety of animals including bees, flies, butterflies, and birds, but coastal populations in Southern California are visited primarily by bumblebees and hummingbirds. Field experiments suggest that plants experience trade-offs in adaptation of corolla length and corolla flare to pollination by hummingbirds versus bumblebees. When hummingbirds are the sole visitors, plants with longer, unflared corollas receive more pollen, but when bumblebees are the primary visitors, plants with intermediate corolla length and flare receive more pollen. Selection by hummingbirds is stronger than that imposed by bumblebees, so that if the contributions of hummingbirds and bumblebees to plant fitness are additive, an asymmetric trade-off results. This additive model predicts that a pollination environment containing both birds and bees should select for a floral phenotype specialized to birds. I will report the results of field and flight cage experiments in which I explore the functional basis of these trade-offs (i.e., whether they stem from preferences by pollinators for particular floral forms or from the mechanical efficiency with which certain floral forms transfer pollen), and in which I test the hypothesis that the contributions of birds and bees to plant fitness are additive.
KEY WORDS: Dudleya spp., trade-off, hummingbird, bumblebee