Mutualists and antagonists mediate frequency-dependent selection on floral display.

Theory predicts that, with conflicting selection pressures mediated by mutualists and antagonists, alternative reproductive strategies can be maintained by negative frequency-dependent selection if it results in rare-morph advantage. We combined field experiments and surveys of natural populations to determine whether selection on floral display is frequency dependent in the self-incompatible herb Primula farinosa, which is polymorphic for inflorescence height and occurs in a short-scaped and a long-scaped morph. Among short-scaped plants, both pollination success, quantified as initiation of fruits and seeds, and seed predation were positively correlated with the relative frequency of the long-scaped morph. The relative strength of these effects and the direction of the resulting frequency-dependent selection on scape morph varied among years and populations. The results suggest that both mutualists and antagonists may mediate frequency-dependent selection and that frequency dependence may vary from positive to negative with rare-morph advantage, depending on the relative strength of these interactions.