A species is the finest standard level, or the basal rank, of etorical 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 worlds bring the total number of classified species into the billions.
The concept of a species is not completely precisely defined, especially with regard to all the different kinds of living things that exist, and that reproduce (or have their numbers replenished) by radically different methods. It is widely believed that, at least among sexually reproducing organisms, 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 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.
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 languages. Regardless, though, it's usually 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 kingdoms.) 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—not as Homo sapiens, not as Homo Sapiens, and not as homo sapiens.
Not all species are of natural origin. New artificial creatures may be created sufficiently different from other organisms to qualify as distinct species. New species may be created by artificial selection, by genetic manipulation, by magic, or perhaps by direct construction. If the artificial creatures are sufficiently similar anatomically, biochemically, and, if applicable, genetically to existing life forms, they may be placed in the same taxa; certain taxa, such as the universe of Machillae, consist entirely of artificial creatures of various sorts.
One scientist, Leigh Van Valen, has even suggested that a cell line frequently used in research, called HeLa after the woman from whom the original cells were taken, 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 or higher taxa this family would belong to.
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.
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 liger, made famous by the movie Napoleon Dynamite, and even with the horse-donkey hybrid the mule, byword of sterility—reputation notwithstanding, some female mules are actually fertile. However, other hybrid types may include fertile male specimens, such as the coyote/dog cross the coydog and the leopard/lion cross the leopon (which, by crossing with a liguar, or lion/jaguar cross, has produced a second-generation hybrid called the leoligulor).
There are even some cases of fertile hybrids of species of entirely different genera. Mammalian examples include the beefalo, a cross between the domestic bovine Bos primigenius and the American bison Bison bison; the cama, a cross between the dromedary Camelus dromedarius and the llama Lama glama; and the Bengal cat, a cross between a housecat, Felis catus, and an Asian leopard cat, Prionailurus bengalensis (or rather the descendant of such a cross—most domestic Bengal cats are at least four generations removed from the actual crossing). The widespread grain triticale is a botanical example of such a hybrid, originating in a cross between wheat (genus Triticum) and rye (genus Secale).
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 genes) of another, or even combining two life forms of different species 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.
Even lacking magic, some unusual hybridizations may be possible through technological methods such as genetic engineering. It may take great knowledge and skill to combine the genotypes 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.
- Species on Wikipedia