Floral isolation between Aquilegia formosa and Aquilegia pubescens

The acquisition of floral nectar spurs is correlated with increased species diversity across multiple clades. We tested whether variation in nectar sp urs influences reproductive isolation and, thus, can potentially promote sp ecies diversity using two species of Aquilegia, Aquilegia formosa and Aquil egia pubescens, which form narrow hybrid zones. Floral visitors strongly di scriminated between the two species both in natural populations and at mixe d-species arrays of individual flowers. Bees and hummingbirds visited flowe rs of A.formosa at a much greater rate than flowers of A.pubescens. Hawkmot hs, however, nearly exclusively visited flowers of A.pubescens. We found th at altering the orientation of A. pubescens flowers from upright to pendent , like the flowers of A.formosa, reduced hawkmoth Visitation by an order of magnitude. In contrast, shortening the length of the nectar spurs of A.pub escens flowers to a length similar to A.formosa flowers did not affect hawk moth visitation. However, pollen removal was significantly reduced in flowe rs with shortened nectar spurs. These data indicate that floral traits prom ote floral isolation between these species and that specific floral traits affect floral isolation via ethological isolation while others affect flora l isolation via mechanical isolation.