A test of the Grant-Stebbins pollinator shift model of floral evolution

Pollinators are thought to play a key role in driving incipient speciation within the angiosperms. However, the mechanisms underlying floral divergence in plants with generalist pollination systems, remains understudied.

Brunsvigia gregaria displays significant geographical variation in floral traits and is visited by diverse pollinator communities. Because pollinators are often shared between populations, we investigated whether specific pollinators are responsible for driving floral divergence between them.

Three distinct ecotypes were identified, each dominated by three different pollinators: bees, swallowtail butterflies, and long-proboscid flies. Across seven populations, we found a pattern of association between style length and the morphology of pollinators that visit the flowers most frequently and contact the reproductive parts most often. Furthermore, we found significant linear, quadratic and correlational selection on flower number, tepal length and style length within the butterfly and bee-dominated populations. We also found partial evidence for divergent selection on these traits between experimental sites.

Our findings suggest that a handful of key pollinators that vary in their importance have the potential to drive population level divergence in floral traits, which may lead to pollination ecotype formation.

Methods
This data was collected using floral measurements at nine localities in the Eastern Cape of South Africa. At seven of these localities, we documented visitation rates and contact with floral reproductive parts to determine pollinator importance. We additionally caught and measured pollinators for their functional body traits. At three of these localities, we also conducted phenotypic selection experiments, in which we measured floral traits from several hundred individuals and female fitness.