Negative frequency-dependent selection maintains a dramatic flower color polymorphism in the rewardless orchid Dactylorhiza sambucina (L.) Soo

The orchid Dactylorhiza sambucina shows a stable and dramatic flower-color polymorphism, with both yellow- and purple-flowered individuals present in natural populations throughout the range of the species in Europe. The evol utionary significance of flower-color polymorphisms found in many rewardles s orchid species has been discussed at length, but the mechanisms responsib le for their maintenance remain unclear. Laboratory experiments have sugges ted that behavioral responses by pollinators to lack of reward availability might result in a reproductive advantage for rare-color morphs. Consequent ly, we performed an experiment varying the relative frequency of the two co lor morphs of D. sambucina to test whether rare morph advantage acted in th e natural habitat of the species. We show here clear evidence from this man ipulative experiment that rare-color morphs have reproductive advantage thr ough male and female components. This is the first demonstration, to our kn owledge, that negative frequency-dependent selection through pollinator pre ference for rare morphs can cause the maintenance of a flower-color polymor phism.