Floral traits are hypothesized to evolve primarily in response to selection by pollinators. However, selection can also be mediatedby other environmental factors. To understand the relative importance of pollinator-mediated selection and its variation amongtrait and pollinator types, we analyzed directional selection gradients on floral traits from experiments that manipulated theenvironment to identify agents of selection. Pollinator-mediated selection was stronger than selection by other biotic factors (e.g.,herbivores), but similar in strength to selection by abiotic factors (e.g., soil water), providing partial support for the hypothesisthat floral traits evolve primarily in response to pollinators. Pollinator-mediated selection was stronger on pollination efficiencytraits than on other trait types, as expected if efficiency traits affect fitness via interactions with pollinators, but other trait typesalso affect fitness via other environmental factors. In addition to varying among trait types, pollinator-mediated selection variedamong pollinator taxa: selection was stronger when bees, long-tongued flies, or birds were the primary visitors than when theprimary visitors were Lepidoptera or multiple animal taxa. Finally, reducing pollinator access to flowers had a relatively small effecton selection on floral traits, suggesting that anthropogenic declines in pollinator populations would initially have modest effectson floral evolution.
