Subspecies

In zoology, under the International Code of Zoological Nomenclature, the subspecies is the only taxonomic rank below that of species that can receive a name. In botany and mycology, under the International Code of Nomenclature for algae, fungi, and plants, other infraspecific ranks, such as variety, may be named. In bacteriology and virology, under standard bacterial nomenclature and virus nomenclature, there are recommendations but not strict requirements for recognizing other important infraspecific ranks.

A taxonomist decides whether to recognize a subspecies. A common criterion for recognizing two distinct populations as subspecies rather than full species is their ability to interbreed even if some male offspring may be sterile. In the wild, subspecies do not interbreed due to geographic isolation or sexual selection. The differences between subspecies are usually less distinct than the differences between species.

Nomenclature

The scientific name of a species is a binomial or binomen, and comprises two Latin words, the first denoting the genus and the second denoting the species. The scientific name of a subspecies is formed slightly differently in the different nomenclature codes. In zoology, under the International Code of Zoological Nomenclature (ICZN), the scientific name of a subspecies is termed a trinomen, and comprises three words, namely the binomen followed by the name of the subspecies. For example, the binomen for the leopard is Panthera pardus. The trinomen Panthera pardus fusca denotes a subspecies, the Indian leopard. All components of the trinomen are written in italics.

In botany, subspecies is one of many ranks below that of species, such as variety, subvariety, form, and subform. To identify the rank, the subspecific name must be preceded by "subspecies" (which is abbreviated to "subsp.", or the unpreferred "ssp."), as in Schoenoplectus californicus subsp. tatora.

In bacteriology, the only rank below species that is regulated explicitly by the code of nomenclature is subspecies, but infrasubspecific taxa are extremely important in bacteriology; Appendix 10 of the code lays out some recommendations that are intended to encourage uniformity in describing such taxa. Names published before 1992 in the rank of variety are taken to be names of subspecies (see International Code of Nomenclature of Prokaryotes). As in botany, subspecies is conventionally abbreviated as "subsp.", and is used in the scientific name: Bacillus subtilis subsp. spizizenii.

Nominotypical subspecies and subspecies autonyms

In zoological nomenclature, when a species is split into subspecies, the originally described population is retained as the "nominotypical subspecies", "nominate subspecies" or "definitive subspecies", which repeats the same name as the species. For example, Motacilla alba alba (often abbreviated M. a. alba) is the nominotypical subspecies of the white wagtail (Motacilla alba).

The subspecies name that repeats the species name is referred to in botanical nomenclature as the subspecies "autonym", and the subspecific taxon as the "autonymous subspecies".

Doubtful cases

When zoologists disagree over whether a certain population is a subspecies or a full species, the species name may be written in parentheses. Thus Larus (argentatus) smithsonianus means the American herring gull; the notation within the parentheses means that some consider it a subspecies of a larger herring gull species and therefore call it Larus argentatus smithsonianus, while others consider it a full species and therefore call it Larus smithsonianus (and the user of the notation is not taking a position).