Salvia - Classification of Species

Classification of Species

George Bentham was first to give a full monographic account of the genus in 1832-1836, and based his classifications on staminal morphology. The defining characteristic of the genus Salvia is the unusual pollination mechanism, which consists of two stamens (instead of the typical four found in other members of the tribe Mentheae and the way the two stamens are connected to form a lever. When a pollinator probes a male stage flower for nectar, the lever causes the stamens to move and the pollen to be deposited on the pollinator. When the pollinator withdraws from the flower, the lever returns the stamens to their original position. In older, female stage flowers, the stigma is bent down in a general location that corresponds to where the pollen was deposited on the pollinator's body. The lever of most Salvia species is not specialized for a single pollinator, but is generic and selected to be easily released by many bird and bee pollinators of varying shapes and sizes. It is believed that this is a key factor in the speciation of this large group of diverse plants.

Bentham's work on classifying the family Labiatae (Labiatarum Genera et Species (1836)) is still the only comprehensive and global organization of the family. While he was clear about the integrity of the overall family, he was less confident about his organization of Salvia, the largest genus in Labiatae (also called Lamiaceae). Based on his own philosophy of classification, he wrote that he "ought to have formed five or six genera" out of Salvia. In the end, he felt that the advantage in placing a relatively uniform grouping in one genus was "more than counterbalanced by the necessity of changing more than two hundred names." At that time there were only 291 known Salvia species.

Bentham eventually organized Salvia into twelve sections (originally fourteen), based on differences in corolla, calyx, and stamens. These were placed into four subgenera that were generally divided into Old World and New World species:

• Subgenus Salvia: Old World (Sections: Hymenosphace, Eusphace, Drymosphace)
• Subgenus Sclarea: Old World (Sections: Horminum, Aethiposis, Plethiosphace)
• Subgenus Calosphace: New World (Section: Calosphace)
• Subgenus Leonia: Old and New World (Sections: Echinosphace, Pycnosphace, Heterosphace, Notiosphace, Hemisphace)

His system is still the most widely studied classification of Salvia, even though more than 500 new species have been discovered since his work. Other botanists have since offered modified versions of Bentham's classification system, while botanists in the last hundred years generally do not endorse Bentham's system.

It was long assumed that Salvia's unusual pollination and stamen structure had evolved only once, and that therefore Salvia was monophyletic, meaning that all members of the genus evolved from one ancestor. However, through DNA sequencing it now appears that somewhat different versions of this lever mechanism have evolved in the tribe Mentheae, and at least three different times within Salvia, making the genus clearly non-monophyletic. The genus may therefore consist of as many as three different clades, or branches.

The description of individual species within Salvia has undergone constant revision. Many species are similar to each other, and many species have varieties that have been given different specific names. There have been as many as 2,000 named species and subspecies. Over time, the number has been reduced to less than a thousand. A modern and comprehensive study of Salvia species was done by Gabriel Alziar, in his Catalogue Synonymique des Salvia du Monde (1989) (World Catalog of Salvia Synonyms). He found that the number of distinct species and subspecies could be reduced to less than 700.