Selection for pollination success and the mechanical fit of Impatiens flowers around bumblebee bodies

Selection on flowers has often been viewed as being particularly strict, constant, and responsible for species differences. Impatiens pallida and I. capensis flowers fit snugly around bees, leading one to expect a close relationship between floral morphology and pollination success. My studies on the amount of pollen removed from androecia and deposited on stigmas in single visits by bumblebees did not confirm this supposition. Trimming off parts of the floral vestibule with scissors and gluing in pleats had very little effect on the amount of pollen that bees moved. In reciprocal transfer experiments, flowers from different populations sometimes differed in the amount of pollen moved, but when the two species were compared in sympatry, pollen removal and deposition differed hardly at all. A comparison of the relationship between pollen movement and floral morphology among 15 populations showed that, although there was great heterogeneity in the amount of pollen moved, the observed differences were independent of floral morphology. None of this supports a belief in strong selection that fine-tunes the mechanical fit between bee and flower; selection for visitation success based on pollinator behaviour may have a much stronger influence on floral characters.