DRIVERS OF ORCHID DIVERSITY. “The evolution of pollinia, specialization on individual pollinators or mycorrhizal fungi, pollination via deceit, euglossine bees or Lepidoptera, epiphytism per se or associated traits such as CAM photosynthesis, and predominant distribution in the tropics (and especially in extensive cordilleras) have all been proposed as drivers of the extraordinary species richness of orchids.”
http://rspb.royalsocietypublishing.org/content/282/1814/20151553