Taxonomy of the Orchidaceae

The taxonomy of the Orchidaceae (orchid family) has evolved slowly during the last 250 years, starting with Carl Linnaeus who in 1753 recognized eight genera.

It reflects the considerable progress in orchid taxonomy that had been made since Dressler published his classification in 1993.

In the 1990s, orchid taxonomy began to be influenced by molecular phylogenetics based on DNA sequences.

[16] The following taxonomy follows largely the classification system of Robert Louis Dressler, an orchid specialist and adjunct curator at the Florida Museum of Natural History.

This comprehensive classification relies heavily on morphology, especially a few key characters, such as anther configuration and pollinarium structure.

Cladistic analyses, especially those based on molecular data, provide a firmer basis for classification than intuition, and the certainty (or uncertainty) of conclusions can be quantified by measures of statistical support.

Some of the first molecular phylogenetic studies of monocots resolved the Orchidaceae as sister to the astelioid clade of the order Asparagales, but this result never had strong statistical support.

[17][18] According to cladistic analyses based on morphological character states or on nucleotide sequences, the orchid family is a monophyletic group.

Dressler's delimitation of subfamilies was contradicted by subsequent studies of mitochondrial, chloroplast, and nuclear DNA sequences.

In 2003, a new phylogenetic classification divided Orchidaceae into five subfamilies: Apostasioideae, Vanilloideae, Cypripedioideae, Orchidoideae, and Epidendroideae.

The subfamilies Orchidoideae and Epidendroideae clearly formed a monophyletic group and Dressler believed that their closest relative was Vanilloideae.

In 2006, a study based on the plastid genes rbcL and atpB found the closest relative of this pair to be Cypripedioideae, rather than Vanilloideae.

Compared to previous classifications, more of the tribes and subtribes of Dressler were monophyletic, but not all of them were supported by subsequent studies.

In the classification that was published in 2015, the authors expressed doubt about their division of the tribes Orchideae and Vandeae into subtribes.

These authors singled out the tribe Podochileae, as well as the subtribes Oncidiinae, Goodyerinae, and Angraecinae as being in special need of phylogenetic study.

All well-sampled molecular phylogenetic studies have produced strong bootstrap support for its position as sister to a clade consisting of the other orchid subfamilies.

Bootstraping is a method of resampling for quantifying the statistical support for nodes in a phylogenetic tree (= a treelike diagram showing the evolutionary diversification of organisms).

Plants with mealy or paste-like pollen, which ordinarily are not aggregated into pellets, called pollinia, with two or three fertile long anthers, leaves with sheathing bases, elongated staminode and labellum similar to the petals.

All show a unique development of the single anther: it is incumbent, meaning that it forms a right angle with the column axis or pointed backward in many genera.

Many of the species develop pseudobulbs (i.e. a bulge at the base of a stem), that are normally shorter and sturdier than those in the epidendroids.

Species of this polyphyletic tribe occur in all continents (except Antarctica), but mainly in North and South America and tropical Asia.