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.

The differences between subspecies are usually less distinct than the differences between species, but more distinct than the differences between races or breeds. 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 [1]. 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 [2] (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 a ternary name.

Nominate subspecies

A subspecies sharing the same name as its species is known as the nominate subspecies. A nominate subspecies is very easily recognizable by the repetition of the species. For example, Motacilla alba alba (often abbreviated Motacilla a. alba) is the nominate subspecies of the White Wagtail (Motacilla alba).

(A somewhat similar system also applies in the case of genera: a genus may have a subgenus of the same name, and that is known as the nominate subgenus, for example the Kerry slug, Geomalacus maculosus, is also sometimes known as Geomalacus (Geomalacus) maculosus.)

Doubtful cases

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 was not taking a position).

Criteria

Members of one subspecies differ morphologically or by different coding sequences of a peptide from members of other subspecies of the species. Subspecies are defined in relation to species.

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. [1]. 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.

A monotypic species has no distinct population or races, or rather one race comprising the whole species. Monotypic species can occur in several ways:

* 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.

References

1. ^ Ernst Mayr, Populations, Species, and Evolution: An Abridgment of Animal Species and Evolution.

* Ernst W. Mayr, Peter D. Ashlock: Principles of Systematic Zoology, Mcgraw-Hill College, 1991, ISBN 0-07-041144-1