The evolution of floral variation without pollinator shifts in Gorteria diffusa (Asteraceae)

One of the most widely accepted explanations for floral diversification in angiosperms is the pollinator-shift model developed by Verne Grant and Ledyard Stebbins. According to this model, the most profound changes in floral traits (such as morphology, color, patterning and scent) occur when plants undergo adaptive shifts between pollinator classes. We tested this model through investigations of geographical variation in floral form and pollinator assemblages in the South African annual daisy Gorteria diffusa. This species has elaborate insect-like ornaments on the capitulum, which attract bee flies belonging to the genus Megapalpus. We found unprecedented levels of geographically structured intraspecific variation and identified 14 discrete forms that vary in the morphology and ornamentation of the capitulum. This variation is not due to phenotypic plasticity because differences among forms were maintained in plants grown from seed in a common garden experiment. Contrary to predictions from the pollinator-shift model, all populations, regardless of floral phenotype, were pollinated primarily by a single species of Megapalpus bee fly. Much of the extensive variation in floral form in G. diffusa therefore appears to have arisen without evolutionary shifts between pollinator types.