Floral isolation between Aquilegia formosa and Aquilegia pubescens
Abstract
The acquisition of floral nectar spurs is correlated with increased species diversity across multiple clades. We tested whether variation in nectar spurs influences reproductive isolation and, thus, can potentially promote species diversity using two species ofAquilegia,Aquilegia formosa andAquilegia pubescens, which form narrow hybrid zones. Floral visitors strongly discriminated between the two species both in natural populations and at mixed-species arrays of individual flowers. Bees and hummingbirds visited flowers ofA. formosa at a much greater rate than flowers ofA. pubescens. Hawkmoths, however, nearly exclusively visited flowers ofA. pubescens. We found that altering the orientation ofA. pubescens flowers from upright to pendent, like the flowers ofA. formosa, reduced hawkmoth visitation by an order of magnitude. In contrast, shortening the length of the nectar spurs ofA. pubescens flowers to a length similar toA. formosa flowers did not affect hawkmoth visitation. However, pollen removal was significantly reduced in flowers with shortened nectar spurs. These data indicate that floral traits promote floral isolation between these species and that specific floral traits affect floral isolation via ethological isolation while others affect floral isolation via mechanical isolation.
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