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In biological classification, subspecies (abbreviated "subsp." or "ssp."; plural: "subspecies") is either a taxonomic rank subordinate to species, or a taxonomic unit in that rank. A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more (including any that are extinct), never just one.
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. 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" do not have names regulated by the ICZN. Such forms have no official ICZN status, though they may be useful in describing altitudinal or geographical clines, pet breeds, transgenic animals, etc. While the scientific name of species is a binomen, the scientific name of a subspecies is a trinomen - a binomen followed by a subspecific name. A tiger's binomen is Panthera tigris, so for a Sumatran tiger the trinomen is, for example, Panthera tigris sumatrae.
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 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 "subsp." or "ssp.", as Schoenoplectus californicus ssp. tatora (Totora). Note that all parts of the binomial are italicised except the actual word "subsp." or "ssp." Any botanical name including a subspecies, variety, etc., is called an infraspecific name.
In zoological nomenclature when a species is split into subspecies, the originally described population is retained as the "nominotypical subspecies" or "nominate subspecies", which repeats the same name as the species. For example, Motacilla alba alba (often abbreviated Motacilla a. alba) is the nominotypical 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).
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Members of one subspecies differ morphologically or by different coding sequences of DNA from members of other subspecies of the species. Subspecies are defined in relation to species.
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.
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.
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