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A '''species''' is the finest standard level, or the basal rank, of [[etory|etorical]] [[alpha taxonomy|taxonomic]] classification, although sometimes species are further subdivided into [[subspecies]].  The species name makes up the latter half of a species' [[scientific name]], which is written as the [[genus]] and then the species, in italics, the former capitalized and the latter lower case.  Over two million species (according to higher estimates, perhaps as many as a hundred million) have been named just on [[True Earth]] (and there may be by some estimates as many as fifty million left unclassified, though other estimates are significantly lower); the species on other [[world]]s bring the total number of classified species into the billions.

==Criteria==
The concept of a species is not completely precisely defined, especially with regard to all the different kinds of [[life form|living thing]]s that exist, and that reproduce (or have their numbers replenished) by radically different methods.  It is widely believed that, at least among [[procreation|sexually reproducing]] [[organism]]s, the members of a species must be mutually interfertile; if a male and female organism can mate with each other and produce fertile offspring, then they belong to the same species.  Contrary to popular belief, however, this is ''not'' the main criterion actually used by biologists.  In fact, there are a number of cases in which it does not match the accepted species classification; there are organisms which ''can'' interbreed and produce fertile offspring but are considered to be of different species or even of different genera, and there are organisms of the same species that for whatever reason are incapable of interbreeding.  Furthermore, it fails to apply to organisms that reproduce asexually, and is rendered problematic by the phenomenon of [[Wikipedia:horizontal gene transfer|horizontal gene transfer]].

In general, for sexually reproducing organisms, a species is more or less defined as a population of organisms that is more or less reproductively isolated, that reproduces exclusively, or almost exclusively, with other members of the same population.  This still leaves some fuzziness, but has proven fairly adequate for a working definition.  For beings that reproduce by other means, the lines between species are even more subjective, and may depend upon the particular [[taxon]].

==Nomenclature==
In standard taxonomy, the species name, like the other taxonomic descriptors, conventionally comes from the [[Latin]] or [[Greek]].  It may be a Latin word or a compound of Latin words; it may include Latinized versions of proper nouns or of words in other [[language]]s.  Regardless, though, it's usually [[grammar|grammatically]] a Latin noun or adjective.  Sometimes the species name looks Latin but doesn't actually mean anything, and is chosen for purposes of euphony or [[wordplay]].

Though many species have unambiguous common names, every species can be referred to uniquely by its [[genus]] and species name, in that order.  (Two different species can have the same species name as long as they are in different genera... though different genera with the same name may also occur, if they're in different [[kingdom (taxonomy)|kingdom]]s.)  By convention, the genus is capitalized, and the species in lower case; both are italicized.  Thus, for example, the [[human]] species is written as ''Homo sapiens''&mdash;not as Homo sapiens, not as ''Homo Sapiens'', and not as ''homo sapiens''.  

==Artificial species==
Not all species are of natural origin.  New [[artificial creature]]s may be created sufficiently different from other organisms to qualify as distinct species.  New species may be created by [[artificial selection]], by [[genetics|genetic]] manipulation, by [[magic]], or perhaps by direct construction.  If the artificial creatures are sufficiently similar [[anatomy|anatomically]], [[biochemistry|biochemically]], and, if applicable, genetically to existing life forms, they may be placed in the same taxa; certain taxa, such as the [[universe (taxonomy)|universe]] of [[Machillae]], consist entirely of artificial creatures of various sorts.

One scientist, [[Wikipedia:Leigh Van Valen|Leigh Van Valen]], has even suggested that a cell line frequently used in research, called [[Wikipedia:HeLa|HeLa]] after the woman from whom the original cells were taken, [[Wikipedia:Henrietta Lacks|Henrietta Lacks]], has so diversified and taken on such an independent existence that it should be considered a separate species, which he called ''Helacyton gartleri''.  This idea has not, however, been generally accepted by the wider biological community, though it hasn't been formally rejected either.  In any case, although Van Valen placed ''Helacyton'' in the proposed [[family]] Helacytidae, it's not at all clear what [[order (taxonomy)|order]] or higher [[taxon|taxa]] this family would belong to.

==Crossing species==
A life form produced by crossing two different species is called a [[hybrid]].  Hybrids generally combine or are intermediate between the characteristics of their parents, though this isn't always true in every particular; certain hybrids, for instance, are larger on the average than either parent species.

===Natural hybrids===
Hybridization of species does sometimes occur in nature, producing viable and possibly fertile offspring.  In fact, it's common for plant hybrids to actually be stronger than the parent varieties.  Largely for this reason, most major grain varieties in use today actually originated as hybrids of related species.  Many other common food plants also originated as hybrids, including the grapefruit, peppermint, and rutabaga.  
			
In the case of animals, often only the females of a hybrid variety will be fertile, and not necessarily all of them.  This is is the case with the lion-tiger hybrid the [[Wikipedia:liger|liger]], made famous by the movie [[Wikipedia:Napoleon Dynamite|Napoleon Dynamite]], and even with the horse-donkey hybrid the [[Wikipedia:mule|mule]], byword of sterility&mdash;reputation notwithstanding, some female mules are actually fertile.  However, other hybrid types may include fertile male specimens, such as the coyote/dog cross the [[Wikipedia:coydog|coydog]] and the leopard/lion cross the [[Wikipedia:leopon|leopon]] (which, by crossing with a liguar, or lion/jaguar cross, has produced a second-generation hybrid called the [[Wikipedia:Panthera hybrid#Leoligulor|leoligulor]]).

There are even some cases of fertile hybrids of species of entirely different genera.  [[Mammal]]ian examples include the [[Wikipedia:beefalo|beefalo]], a cross between the domestic bovine ''Bos primigenius'' and the American bison ''Bison bison''; the [[Wikipedia:cama (animal)|cama]], a cross between the dromedary ''Camelus dromedarius'' and the llama ''Lama glama''; and the [[Wikipedia:Bengal (cat)|Bengal cat]], a cross between a housecat, ''Felis catus'', and an Asian leopard cat, ''Prionailurus bengalensis'' (or rather the descendant of such a cross&mdash;most domestic Bengal cats are at least four generations removed from the actual crossing).  The widespread grain [[Wikipedia:triticale|triticale]] is a botanical example of such a hybrid, originating in a cross between wheat (genus ''Triticum'') and rye (genus ''Secale'').

===Magical hybrids===
Species that cannot hybridize naturally may still be crossable through [[magic]].  This doesn't necessarily mean allowing one species to bear the young of another, though this may be possible.  It may also mean directly infusing one species with some qualities (and perhaps some [[gene]]s) of another, or even combining two life forms of different species [[henosis|henotically]] into a single entity.  In any case, in principle, it may be possible to hybridize any two species through magic, though the results of such hybridizations may be unpredicatable.

===Technological hybrids===
Even lacking magic, some unusual hybridizations may be possible through [[technology|technological]] methods such as [[genetic engineering]].  It may take great knowledge and skill to combine the [[genotype]]s of two animals in such a way as to eliminate their incompatibilities and end up with a viable genetic code, but that doesn't mean it's impossible.  For someone who knows what he's doing, technological hybridization may be a highly customizable process, with the engineer able to choose which characteristics to retain or combine from each of the parent organisms.

==See also==
*[[Wikipedia:Species|Species]] on Wikipedia
[[Category:Taxonomy]]