Temporal variation in the selection on floral traits in Cyclopogon elatus (Orchidaceae)

摘要

Over the years, selection can vary in intensity and direction. Selection on traits related to the mechanical fit with pollinators is expected to vary according to changes in pollinator assemblage. Provided that pollinators do not change over time selection on traits related to attraction, such as floral fragrance, is expected to be mainly affected by environmental conditions because its production is resource limited. We examined selection patterns on osmophore area (as a surrogate of odour production), nectary depth and flower number in the orchid Cyclopogon elatus during four consecutive years, and simultaneously recorded variations in weather conditions and differences in pollinator assemblages. We also studied whether the osmophore area was positively related to pollinator attraction and examined its possible trade-off with phenology. We found that patterns of selection on nectary depth did not vary significantly among years. This lack of temporal variation is consistent with the stability of a single bee species as the predominant pollinator. On the contrary, selection on osmophore area varied markedly and was negative during the driest year. Negative selection indicates that although pollinators visited plants with larger osmophore areas more frequently, it did not translate into higher fitness. Plants with larger osmophore areas begin to flower earlier and might be more affected by weather conditions, not directly due to resource availability but indirectly because of the lack of pollinators at the beginning of the flowering season.