Trinomial nomenclature

Trinomial nomenclature is used in biology when the organisms within a species fall into separate groupings that need to be distinguished even though they could interbreed, so that the usual binomial nomenclature does not suffice to identify an organism adequately.

The hierarchy of taxa below the species level is:

• subspecies
• variety
• subvariety
• form

Not all are necessarily used for any particular species. In many cases the sub specific groupings are regional. Because of geographical isolation and adaptation to the different circumstances the regional population evolves and over time may become a separate species.

The simplest form of trinomial nomenclature occurs when only a subspecies is being specified. In this case, the subspecies name is added after the species name, without capitalisation but normally typeset in italics. If the genus and species name have already been mentioned in the same paragraph, they are often abbreviated to initial letters: for example one might write, "The Great Cormorant Phalacrocorax carbo has a distinct subspecies in Australasia, the Black Shag P. c. novaehollandiae".

If taxa below the subspecies level are used (which most often arises with plants), an identifying abbreviation is always added. In groups where such lower level divisions are common, it is wiser to indicate when a subspecies name is used, also.

From a scientific point of view, a name is incomplete without an author label and publication details. This specifies who published the name and in what publication and gives the date of the publication in which the name was published.

Examples

• Phalacrocorax carbo novaehollandiae Stephens, 1826 (no abbreviation is included with animal trinomials, novaehollandis is assumed to be a subspecies, and was described by Stephens in 1826)
• Pinus nigra J. F. Arnold subsp. salzmannii (Dunal) Franco (in plants, it is important to specifiy that salzmannii is a subspecies, and identifying J. F. Arnold as the author who first described the speciesEuropean black pine, Dunal as the author who described the species Pinus salzmanii, and Franco as the author who concluded that Pinus salzmanii was only distinct from Pinus nigra at the rank of subspecies)
• Pinus nigra J. F. Arnold var. pallasiana (Lambert) Asch. & Graebn. (if no subspecies name is given, it is assumed to belong to a typical subspecies, in this case Pinus nigra subsp. nigra; and pallasiana is a variety; initially described by Lambert as a species (Pinus pallasiana) and subsequently reduced to a variety of Pinus nigra subsp. nigra).
• Pinus nigra J. F. Arnold subsp. salzmannii var. corsicana (Loudon) Hylander (the Corsican pine is a variety of the western subspecies of the European black pine).
• Adenia aculeata subsp. inermis de Wilde (specifiying that inermis is a subspecies, and identifying de Wilde as the author who named this organism)
• Acanthocalycium klimpelianum var. macranthum (Rausch) J.G.Lamb. (no subspecies name required; macranthum is a variety; initially described by Rausch as a Lobivia and subsequently entered into the genus Acanthocalycium by J.G. Lambert).
• Astrophytum myriostigma subvar. glabrum Backeb. (glabrum is a subvariety name)
• Acanthocalycium spiniflorum forma klimpelianum (Weidlich & Werderm.) Donald (klimpelianum is a form name).