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Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification
Authors:RAYMOND L TREMBLAY  JAMES D ACKERMAN  JESS K ZIMMERMAN  RICARDO N CALVO
Institution:Department of Biology, 100 carr. 908, University of Puerto Rico-Humacao, Humacao, Puerto Rico 00971–4300; Department of Biology, PO Box 23360, University of Puerto Rico-Rio Piedras, San Juan, Puerto Rico 00931–3360; Institute for Tropical Ecosystem Studies, University of Puerto Rico, PO Box 21910, San Juan, Puerto Rico 00931–1910; Department of Biology, University of Miami, Coral Gables, FL 33124, USA
Abstract: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.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 1–54.
Keywords:cost of reproduction  fruit set  gene flow  genetic drift  natural selection  Orchidaceae  pollinator limitations  resource limitation  speciation
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