Subspecies

In zoology, as in other branches of biology, subspecies is the taxonomic rank immediately subordinate to a species. A subspecies is a taxonomic group which is less distinct than the primary stock or species from which it originates. The characteristics attributed to subspecies are generally derived from changes that have taken place or evolved as a result of geographical distribution or isolation from the primary species or nominate form (see below), also called nominate subspecies. In layman's terms, a subspecies may also be described as a diversification of the primary species since a subspecies always has the nominate form or primary species as its common ancestor, i.e. it always originates from a common ancestral stock.

Nomenclature
In zoology, the scientific name of a subspecies is the binomen followed immediately by a subspecific name, e.g. Homo sapiens sapiens. The International Code of Zoological Nomenclature (4th edition, 2000) does not attempt to codify any "infrasubspecific entities" (e.g. human races or pet breeds).

If there is a need for a subspecific taxon in animal nomenclature, a trinomen may be described for a subspecies. Many other "typical specimens" may be described, but these should not be considered as being absolute, unconditional or categorical. These forms have no official status, though they may be useful in describing altitudinal or geographical clines.

A subspecies indicated by the repetition of the specific name is known as the nominate subspecies. Thus Motacilla alba alba is the nominate subspecies of White Wagtail (Motacilla alba). In scientific papers, subspecies is commonly abbreviated subsp. or ssp. — for example, ''White Wagtail ssp. yarrellii'', which is the same as the Pied Wagtail.

Maybe the best known examples are:
 * the dog — Canis lupus familiaris. Compare with Canis lupus dingo;
 * the cat — Felis silvestris catus. Compare with Felis silvestris libyca.

Note:
 * In botany, both subspecies, variety, subvariety, form, and subform are ranks that will get a ternary name.
 * In bacteriology, the terms subspecies and variety are usually interchangeable (see International Code of Nomenclature of Bacteria).

Criteria
Members of one subspecies differ morphologically or by different DNA sequences from members of other subspecies of the species. Subspecies are defined in relation to species. It is not possible to understand the concept of a subspecies without first grasping what a species is. In the context of many large living organisms like trees, flowers, birds, fish and humans, a species can be defined as a distinct and recognisable group that satisfies two conditions:


 * 1) Members of the group are reliably distinguishable from members of other groups. The distinction can be made in any of a wide number of ways, such as: differently shaped leaves, a different number of primary wing feathers, a particular ritual breeding behaviour, relative size of certain bones, different DNA sequences, and so on. There is no set minimum 'amount of difference': the only criterion is that the difference be reliably discernable. In practice, however, very small differences tend to be ignored.
 * 2) The flow of genetic material between the group and other groups is small and sometimes can be expected to remain so because even if the two groups were to be placed together they would not interbreed to any great extent.

Note the key qualifier above: to be regarded as different groups rather than as a single varied group, the difference must be distinct, not simply a matter of continuously varying degree. If, for example, the population in question is a type of frog and the distinction between two groups is that individuals living upstream are generally white, while those found in the lowlands are black, then they are classified as different groups if the frogs in the intermediate area tend to be either black or white, but a single, varied group if the intermediate population becomes gradually darker as one moves downstream.

This is not an arbitrary condition. A gradual change, called a cline, is clear evidence of substantial gene flow between two populations. A sharp boundary between black and white, or a relatively small and stable hybrid zone, on the other hand, shows that the two populations do not interbreed to any great extent and are indeed separate species. Their classification as separate species or as subspecies, however, depends on why they do not interbreed.

If the two groups do not interbreed because of something intrinsic to their genetic make-up (perhaps black frogs do not find white 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 was 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 monotypic species has no 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.

A polytypic species has two or more races or subspecies. 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 races: they are separate species.