NEGATIVE FREQUENCY-DEPENDENT SELECTION BY POLLINATORS ON ARTIFICIAL FLOWERS WITHOUT REWARDS

Many species of nonmodel deceptively pollinated orchids are polymorphi c for corolla color These species are pollinated by naive insects sear ching for nectar, and are not mimics. It has been suggested that the f oraging behavior of insect pollinators during the avoidance learning p rocess results in these stable corolla color polymorphisms; for this t o occur pollinators must induce negative frequency-dependent selection on corolla color. Therefore the hypothesis that pollinator behavior r esults in a preference for rare color morphs of deceptive species was tested experimentally. Bumblebees (Bombus terrestris) foraged in the l aboratory on arrays of artificial flowers with different corolla color morphs. Morphs were varied in frequency, and bumblebee preferences we re recorded on arrays where morphs did and did not contain sucrose sol ution rewards. Bumblebees preferred the most common color morph when f lowers contained sucrose solution rewards, but overvisited rare morphs when sampling dowers that contained no rewards. Bumblebees also tende d to move between unlike color morphs when these were unrewarding, sug gesting that a probabilistic sampling strategy was adopted. Thus exper iments demonstrated that pollinator behavior could result in a selecti ve advantage for rare color morphs of plant species that are pollinate d by deception without mimicry, which would induce negative frequency- dependent selection on corolla color. The observed pollinator behavior could allow stable corolla color polymorphisms to be maintained by se lection in nonmodel deceptively pollinated species.