Pollinator-mediated selection in a specialized pollination system: matches and mismatches across populations.

Most studies on pollinator-mediated selection have been performed in generalized rather than specialized pollination systems. This situation has impeded evaluation of the extent to which selection acts on attraction or specialized key floral traits involved in the plant-pollinator phenotypic interphase. We studied pollinator-mediated selection in four populations of Nierembergia linariifolia, a self-incompatible and oil-secreting plant pollinated exclusively by oil-collecting bees. We evaluated whether floral traits experience variable selection among populations and whether attraction and fit traits are heterogeneously selected across populations. Populations differed in every flower trait and selection was consistently observed for corolla size and flower shape, two traits involved in the first steps of the pollination process. However, we found no selection acting on mechanical-fit traits. The observation that selection occurred upon attraction rather than mechanical-fit traits, suggests that plants are not currently evolving fine-tuned morphological adaptations to local pollinators and that phenotypic matching is not necessarily an expected outcome in this specialized pollination system.