Phylogenetic relationships among all subfamilies and nearly all tribes of Orchidaceae were investigated in a continuing effort to understand the systematics and classification of this large and diverse family. DNA sequences of five plastid genes: rbcL, atpB, psaB, ndhF, and matK from more than 35 orchid genera and one outgroup taxon from Hypoxidaceae were analyzed using parsimony. Data analyses were executed separately for each gene and in combination. The combined data matrix consists of more than 242,000 individual nucleotide characters, making it one of the largest molecular data matrices for Orchidaceae analyzed to date. Trees from individual gene matrix analyses are generally congruent in terms of intersubfamilial and intertribal relationships, but vary in overall resolution, bootstrap support for particular clades, and especially for relationships among the genera of Epidendroideae. The combined five-gene tree, however, is well resolved and supported, especially when successive weighting is applied to the matrix. Epidendroideae and most of its tribes are monophyletic. The subfamily is sister to Orchidoideae (which includes the spiranthoid orchids); Cypripedioideae is sister to this pair of monandrous subfamilies. Vanilloideae is sister to these three and Apostasioideae is sister to the entire remainder of the family. With this limited level of taxon sampling, branch lengths are long among all the taxa except for the higher epidendroid orchids. The tree topology indicates that this particular lineage of mostly epiphytic orchids underwent a rapid evolutionary radiation with high rates of subsequent speciation, perhaps in quite recent geologic history, as has been suggested previously.

Key words: evolution, molecular systematics, orchid, Orchidaceae, phylogeny, plastid