Format: ORAL

Arjan Engelen1, Allan G. Ellis1

Stellenbosch University, Western Cape, South Africa

While pollinator-mediated selection undoubtedly contributes to diversification of plants with specialised pollination phenotypes, its importance in generalist plants is less clear. The adaptive wandering model of floral diversification posits that floral divergence in generalists, occurring in response to selection across quantitative pollinator mosaics, does not result in the trade-off in pollinator use (i.e. ecological specialisation) that is the hallmark of the alternate, pollinator-shift model. Here we test the importance of these alternate mechanisms in generating the striking spatially structured floral variation that characterises the annual Dimorphotheca daisy complex in South Africa. First we show that the system is underlain by a weak spatial pollinator mosaic because all morphotypes interact strongly with a core group of widespread visitors, with use of these pollinators tracking their relative availability in communities, both within and across morphotype ranges. However, each morphotype is associated with some unique and effective pollinating species that are distributed in a largely qualitative spatial mosaic corresponding to floral morphotype distributions. Field experiments with model inflorescences suggest that, while the core pollinators are not exerting selection, the unique species in the assemblage are selecting on divergent trait phenotypes. Although evidence of trade-offs in pollinator use is limited, divergent floral phenotypes do produce weak but significant pollinator partitioning, and thus ethological isolation, in sympatry. While these results largely align with the adaptive wandering model of floral divergence, the fact that spatially discrete floral variation is underlain by largely qualitative gradients in the availability of a small subset of effective pollinators exerting selection on divergent floral traits is akin to the pollinator-shift model. Against the background of a core widespread generalist pollinator community, our results suggest that relative fitness gains associated with traits improving attraction and effective use of additional range-restricted pollinator species may explain the evolution of floral diversity in Dimorphotheca.