Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification

The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinators driven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selection is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and asking which aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. We reaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruit set is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that do not. Reproductive success is skewed towards few individuals in a population and effective population sizes are often small. Population structure, reproductive success and gene flow among populations suggest that in many situations genetic drift may be as important as selection in fostering genetic and morphological variation in this family. Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversity in this family is largely a consequence of sequential and rapid interplay between drift and natural selection.