Subspecies
|
|
In taxonomy, a subspecies is the taxon immediately subordinate to a species. Members of one subspecies differ morphologically and genetically (disputed — see talk page) from members of other subspecies of the species.
| Contents |
Conventions
Conventions regarding infra-specific categories vary between biological disciplines as follows:
Zoology
In zoology, the scientific name of a subspecies is the binomen followed immediately by a subspecific epithet, e.g. Homo sapiens sapiens. The International Code of Zoological Nomenclature (4th edition, 2000) does not attempt to codify any "infrasubspecific entities".
Bacteriology
In bacteriology, the terms subspecies and variety are usually interchangeable.
Botany
In botany, different variations within a species are denominated explicitly as subspecies (subsp.), varieties (var.) or forms (f.); a species may be divided into one or more subspecies, with the subspecies further subdivided into one or more varieties, e.g. European black pine (Pinus nigra):
- Pinus nigra subsp. nigra in the east of the species' range, from Austria and north-east Italy, east to the Crimea and Turkey
- Pinus nigra subsp. nigra var. nigra Austrian pine
- Pinus nigra subsp. nigra var. caramanica Turkish black pine
- Pinus nigra subsp. nigra var. pallasiana Crimean pine
- Pinus nigra subsp. nigra var. nigra Austrian pine
- Pinus nigra subsp. salzmannii in the west of the species' range, from peninsular Italy to Spain and north Africa
- Pinus nigra subsp. salzmannii var. salzmannii Cevennes pine
- Pinus nigra subsp. salzmannii var. corsicana Corsican pine
- Pinus nigra subsp. salzmannii var. mauretanica Atlas Mts black pine
- Pinus nigra subsp. salzmannii var. salzmannii Cevennes pine
Note that one of the taxa always repeats the same name as for the species as a whole; this is referred to as the type or nominate subspecies and variety, and includes the specimen the species was originally described from.
Variety has often mistakenly been used as a synonym of subspecies, but the two ranks are nomenclaturally discrete under the ICBN. Variety is used for lower degrees of difference than subspecies. A form is usually used to designate a minor variation within a population or region. For instance, white-flowered forms of species that usually have coloured flowers are often designated as "f. alba".
A cultivated variant is identified by quoting the cultivar epithet. For example:
- Clematis alpina 'Ruby' is an infraspecific cultivar;
- Magnolia 'Elizabeth' is a hybrid formed from at least two species.
Criteria
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 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:
- 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.
- The flow of genetic material between the group and other groups is small and 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.
Note that the distinction between a species and a subspecies depends only on the likelihood that (absent 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.
It should be noted that if a subspecies is indicated by the repetition of the specific name, it is known as the nominate subspecies. Thus Motacilla alba alba is the nominate subspecies of White Wagtail, Motacilla alba.
Important difference between species and subspecies
Subspecies: a taxonomic subdivision of a species; a group of the organisms, which differ from other members of their species by genetically-encoded morphological and physiological characteristics, and by behavior. Members of different subspecies of the same species are potentially capable of breeding with each other, and production of fertile offspring. However, animals of the different subspecies of the same species might not interbreed even if geographical factor is removed. Differences in appearance and behavior rather often prevent the potential sex partners from recognizing each other as the sex partners. This is especially true for animals with complicated sexual rituals. Members of different species are incapable of reproduction, or produce an infertile offspring.