Orchids are the most diverse family of angiosperms, with over 25 000 species, more than mammals, birds and reptiles combined. Tests of hypotheses to account for such diversity have been stymied by the lack of a fully resolved broad-scale phylogeny. Here, we provide such a phylogeny, based on 75 chloroplast genes for 39 species representing all orchid subfamilies and 16 of 17 tribes, time-calibrated against 17 angiosperm fossils. A supermatrix analysis places an additional 144 species based on three plastid genes. Orchids appear to have arisen roughly 112 million years ago (Mya); the subfamilies Orchidoideae and Epidendroideae diverged from each other at the end of the Cretaceous; and the eight tribes and three previously unplaced subtribes of the upper epidendroids diverged rapidly from each other between 37.9 and 30.8 Mya. Orchids appear to have undergone one significant acceleration of net species diversification in the orchidoids, and two accelerations and one deceleration in the upper epidendroids. Consistent with theory, such accelerations were correlated with the evolution of pollinia, the epiphytic habit, CAM photosynthesis, tropical distribution (especially in extensive cordilleras), and pollination via Lepidoptera or euglossine bees. Deceit pollination appears to have elevated the number of orchid species by one-half but not via acceleration of the rate of net diversification. The highest rate of net species diversification within the orchids (0.382 sp sp 21 My 21) is 6.8 times that at the Asparagales crown.

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All species of the Neotropical subtribes Stanhopeinae and Catasetinae (Orchidaceae) are pollinated exclusively by male euglossine bees which are attracted to and collect the floral fragrances. The orchid-euglossine bee relationship is often highly specific: the flower of a given species of plant may attract males of only one or a few species out of dozens of euglossine species in the habitat. This pollinator specificity is based upon species-specific combinations of floral fragrance compounds which attract only one or a few species of euglossine bees. Such pollinator specificity is an important reproductive isolating mechanism between sympatric interfertile species of orchids. The male bees are thought to use the collected floral fragrance compounds in their own reproductive biology, probably as precursors of their own sex pheromones.

Abstract. Crassulacean acid metabolism (CAM) is a specialised mode of photosynthesis that improves atmospheric CO 2 assimilation in water-limited terrestrial and epiphytic habitats and in CO 2 -limited aquatic environments. In contrast with C 3 and C 4 plants, CAM plants take up CO 2 from the atmosphere partially or predominantly at night. CAM is taxonomically widespread among vascular plants and is present in many succulent species that occupy semiarid regions, as well as in tropical epiphytes and in some aquatic macrophytes. This water-conserving photosynthetic pathway has evolved multiple times and is found in close to 6% of vascular plant species from at least 35 families. Although many aspects of CAM molecular biology, biochemistry and ecophysiology are well understood, relatively little is known about the evolutionary origins of CAM. This review focuses on five main topics: (1) the permutations and plasticity of CAM, (2) the requirements for CAM evolution, (3) the drivers of CAM evolution, (4) the prevalence and taxonomic distribution of CAM among vascular plants with emphasis on the Orchidaceae and (5) the molecular underpinnings of CAM evolution including circadian clock regulation of gene expression.