Orchid evolution and phylogeny

Many possible sources for orchids have been suggested, however, the Hypoxidaceae family (or plants similar to them), which are already extinct, seem more likely to be their parents. Despite being the most diverse family of angiosperms on land, orchids do not have an adequate fossil record, as many aspects of their evolutionary history remain hidden.

In 2007, however, the Dominican Republic Polinario reported the finding of an orchid (the one that was called Meliorchis caribea) preserved in amber and attached to the mesoescuto of an extinct species of bee (Proplebeia dominicana). This fossil is from the Miocene, about 15 to 20 million years ago.

This finding is not only the first orchid fossil, but also the discovery of the first fossil history of interactions between plants and their pollinators. Furthermore, this finding, coupled with cladistic analysis of morphological data, indicates that the most recent ancestor of existing orchids lived in the Cretaceous period, 76 to 84 million years ago.

The monophyly of the orchid is supported both by the morphology and by DNA sequence analysis (Dressler 1981, 1993, and 1995; Chase Burns-Balogh and Funk 1986; Judd et al. 1993; Chase et al. 2000; Fay et al. 2000; Freudenstein et al. 2004).

Also, the phylogenetic relationships within the family, a source of active research in recent decades which still continues, have been elucidated by cladistic analysis of morphology and conserved DNA sequences (Dressler 1986 and 1988, Chase 1983 and 1993, Chase and Chase Hills 1992 and 1992, Cameron Palmer et al. 1999, Kores et al. 2000, Whitten et al. 2000, Salazar et al. 2003, Burns-Balogh and Funk 1986 and 2006, Dressler 1986 and 1993, Cameron Judd et al. 1993, Dressler and Chase 1995, Cameron et al. Freudenstein and Rasmussen 1999 and 1999, Cameron Chase 2000, Freudenstein et al. 2000 and 2004, Molvray et al. 2000, and van den Berg et al. 2005).

Apostasy and Neuwiedia genres (belonging to the subfamily Apostasioideae) are considered sisters to the rest of the members of the family (Dressler 1993, Judd et al. 1993, Dressler and Chase 1995, Neyland and Urbatsch 1996, Cameron et al. 1999, and Freudenstein and Rasmussen 1999).

These two genera, especially Neuwiedia, have retained many ancestral characters, such as the presence of two (Apostasy) or three (Neuwiedia) stamens in their flowers, which are only slightly fused to the style and pollen grains, independent of yes and no stickiness. The other orchids, however, have sticky pollen or pollen grains fused together at the moment they are released from the stamens.

Within the group of orchids with sticky pollen, and the subfamilies Cypripedioideae and Vanilloideae, are basal clades. The first of them (including, for example, Cypripedium and Paphiopedilum) is clearly monophyletic and members share some unique characters, like the lip shaped sac (in the shape of a shoe) and the anther in a modified median shield-shaped staminode, and two functional stamens. Vanilloideae (containing for example, Vanilla, Cleistes and Pogonias) is distinguished as the flowers have only one functional stamen and do not have pollinia.

The remaining subfamilies have only one functional stamen; pollen grains are grouped together forming pollinia and filament yarn is completely fused with the style (Dahlgren et al. 1985, Burns-Balogh and Funk 1986, Judd et al. 1993). This clade, which includes all species with pollen in clustered pollinia, has flowers with only one functional stamen (the species being monandra) and the two lateral stamens are either transformed into slender staminodes or completely missing.

The analysis of morphology (Freudenstein and Rasmussen 1999) and some molecular analysis (Cameron et al. 1999. Molvray et al. 2000) indicate that monandra orchids are monophyletic, however, other molecular analyses (Cameron, Cameron and Chase 2006.and 2000, Freudenstein et al. 2004) argue that reducing the number of functional stamens has occurred twice during the evolution of the family.

Among orchids with pollinia, monandra recognize two major subfamilies, Orchidoideae and Epidendroideae (including Spiranthoideae). Epidendroideae members share the apomorphies of presenting a beaked and incumbent anther (i.e., the anther is bent over the apex of the spine), while members of Orchidoideae share the apomorphies of a sharp apex of the anther, soft stems and convoluted leaves.