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The Neotropical orchid genus Oncidium includes more than 400 known species, at least in its broadest definition. However, the limits of the genus have been a matter of controversy for many years. Chase and Palmer (1992, cited in Chase et al. 2009) first showed that Oncidium as commonly circumscribed was polyphyletic (i.e., the traits traditionally used to place species as members of this genus are shared because they evolved several times independently, not because of shared ancestry). Subsequent studies have confirmed and extended this early work and progress has been made in redefining genera--splitting some species out of Oncidium and merging others--to make Oncidium monophyletic (a monophyletic group being one that includes all [and only] the descendant species of a single common ancestor). (Chase et al. 2009 and references therein)
Oncidium orchids are mainly epiphytic. They are found from Mexico and southern Florida (U.S.A.) through Central America and the Caribbean to Brazil, Bolivia, and Peru. The two centers of diversity are in Mexico and the Andes (according to the recent analysis by Chase et al. , just a single species is known from Brazil). Various species occur from near sea level up to around 2500 meters in elevation, in both exposed situations and dense shade.
It appears that Oncidium as a genus has historically been defined by flower characteristics directly related to its pollination biology (pollination by Centris and possibly other bees). Chase et al. (2009) note that it is unsurprising that natural selection should have resulted in some convergence in these floral traits among several independent lineages, making Oncidium (as traditionally defined) polyphyletic. They advocate minimizing reliance on floral characters in circumscribing genera within the Oncidiinae (the group including Oncidium and related genera) and instead focusing on chromosome number and vegetative characters, such as the structure of pseudobulbs.
Oncidium orchids are widely cultivated for the cut flower trade. The confusing historical taxonomic treatment of this group, combined with very extensive hybridizing by breeders, makes identification of Oncidium species and varieties very challenging, With this in mind, Wu et al. (2010) sequenced the chloroplast genome of a cultivated Oncidium and explored chloroplast sequence variation in a number of other Oncidium varieties. They concluded that, although additional data from the nuclear genome are clearly desirable, data from the chloroplast genome hold great promise for phylogenetic and evolutionary studies in Oncidium breeding and variety identification.