Pollinator specialisation fails to explain rapid speciation in terrestrial orchids

Jamie B Thompson , Eric Robert Hagen, Elizabeth Anne Forward, Oscar Alejandro Pérez-Escobar, James W Clark, James Ackerman, Nicholas Kiefer Priest

Pollinator attraction is central to the reproductive biology and ecology of flowering plants, and pollinator specialisation has long been thought of as a driving force of species generation. Orchids are central to this idea, which dates back to Darwin’s work on pollinator-driven floral evolution. However, most macroevolutionary evidence for the speciation hypothesis comes from studies of genera or tribes, leaving broad-scale patterns unresolved. Here, we reconstruct the evolution of pollination strategy in the species-rich terrestrial orchid subfamily Orchidoideae, and test whether speciation rate is shaped by pollinator specialisation. We identify numerous transitions among reward-based, deception-based, and autonomous pollinator attraction strategies, but along evolutionarily constrained pathways. Curiously, we find that speciation rates are not significantly impacted by pollinator specialisation, a result that is robust to differences both in methodological approaches and definitions of specialisation. Despite shaping ecological interactions and microevolutionary divergence, pollinator specialisation does not influence rates of speciation on macroevolutionary scales. Our findings support a growing view that the origins of plant biodiversity involve complex interactions between traits, ecological opportunities, and environmental contexts, rather than by single force in isolation.