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 know
examples are:
Note:
Members of one subspecies
differ morphologically 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 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:
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.