Subspecies (commonly abbreviated subsp. or ssp.) in biological classification, is either a taxonomic rank subordinate to species, or a taxonomic unit in that rank (plural: subspecies). A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one. (However, all but one subspecies may be extinct, as in Homo sapiens sapiens.)
Organisms that belong to different subspecies of the same species are capable of interbreeding and producing fertile offspring, but they often do not interbreed in nature due to geographic isolation or other factors. The differences between subspecies are usually less distinct than the differences between species, but more distinct than the differences between breeds or races (races can be assigned to different subspecies if taxonomically different). The characteristics attributed to subspecies generally have evolved as a result of geographical distribution or isolation.
In zoology, the International Code of Zoological Nomenclature (4th edition, 1999) accepts only one rank below that of species, namely the rank of subspecies . Other groupings, "infrasubspecific entities" (e.g. pet breeds and Transgenic Animals) do not have names regulated by the ICZN. Such forms have no official status, though they may be useful in describing altitudinal or geographical clines. The scientific name of a subspecies is a binomen followed by a subspecific name, as Panthera tigris sumatrae (Sumatran Tiger). A name of this kind is called a trinomen.
Likewise in bacteriology, the only rank allowed below species is subspecies. Names published before 1992 in the rank of variety are taken to be names of subspecies  (see International Code of Nomenclature of Bacteria).
In botany, subspecies is one of many ranks below that of species, such as variety, subvariety, form, and subform. The subspecific name is preceded by "ssp." or "subsp.", as Schoenoplectus californicus ssp. tatora (Totora). Any botanical name including a subspecies, variety, etc., is called an infraspecific name.
Nominate subspecies and subspecies autonyms
In zoological nomenclature when a species is split into subspecies, the originally described population is retained as the "nominate subspecies", which repeats the same name as its species, for example, Motacilla alba alba (often abbreviated Motacilla a. alba) is the nominate subspecies of the White Wagtail (Motacilla alba).
When biologists 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 with 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).
If the two groups do not interbreed because of something intrinsic to their genetic make-up (perhaps green frogs do not find red frogs sexually attractive, or they breed at different times of year) then they are different species.
If, on the other hand, the two groups would interbreed freely provided only that some external barrier were removed (perhaps there is a waterfall too high for frogs to scale, or the populations are far distant from one another) then they are subspecies. Other factors include differences in mating behavior or time and ecological preferences such as soil content.
Note that the distinction between a species and a subspecies depends only on the likelihood that in the absence of external barriers the two populations would merge back into a single, genetically unified population. It has nothing to do with 'how different' the two groups appear to be to the human observer.
As knowledge of a particular group increases, its categorisation may need to be re-assessed. The Rock Pipit was formerly classed as a subspecies of Water Pipit, but is now recognised to be a full species. For an example of a subspecies, see Pied Wagtail.
Cryptic species are morphologically similar, but have differences in DNA or other factors.
Monotypic and polytypic species
A polytypic species has two or more subspecies, races or more generally speaking, populations that need a separate description. These are separate groups that are clearly distinct from one another and do not generally interbreed (although there may be a relatively narrow hybridization zone), but which would interbreed freely if given the chance to do so. Note that groups which would not interbreed freely, even if brought together such that they had the opportunity to do so, are not subspecies: they are separate species.
- All members of the species are very similar and cannot be sensibly divided into biologically significant subcategories.
- The individuals vary considerably but the variation is essentially random and largely meaningless so far as genetic transmission of these variations is concerned.
- The variation among individuals is noticeable and follows a pattern, but there are no clear dividing lines among separate groups: they fade imperceptibly into one another. Such clinal variation always indicates substantial gene flow among the apparently separate groups that make up the population(s). Populations that have a steady, substantial gene flow among them are likely to represent a monotypic species even when a fair degree of genetic variation is obvious.
- Ernst W. Mayr, Peter D. Ashlock: Principles of Systematic Zoology, Mcgraw-Hill College, 1991, ISBN 0-07-041144-1
Taxonomic ranks Magnorder Domain/Superkingdom Superphylum/Superdivision Superclass Superorder Superfamily Supertribe Superspecies Kingdom Phylum/Division Class Legion Order Family Tribe Genus Species Subkingdom Subphylum Subclass Cohort Suborder Subfamily Subtribe Subgenus Subspecies Infrakingdom/Branch Infraphylum Infraclass Infraorder Section Infraspecies Microphylum Parvclass Parvorder Series Variety Form
Wikimedia Foundation. 2010.