Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding     and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or morphology. Presence of specific locally adapted traits may further subdivide species into subspecies.

Biologists' working definition
A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. Extinct species known only from fossils are generally difficult to give precise taxonomic rankings to.

Biologists view species as statistical phenomena and not as categories or types. This view is counterintuitive since the classical idea of species is still widely-held, with a species seen as a class of organisms exemplified by a "type specimen" that bears all the traits common to this species. Instead, a species is now defined as a separately evolving lineage that forms a single gene pool. Although properties such as genetics and morphology are used to help separate closely-related lineages, this definition has fuzzy boundaries. However, the exact definition of the term "species" is still controversial, particularly in prokaryotes, and this is called the species problem. Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned.

Common names and species
The commonly used names for plant and animal taxa sometimes correspond to species: for example, "lion", "walrus", and "Camphor tree" – each refers to a species. In other cases common names do not: for example, "deer" refers to a family of 34 species, including Eld's Deer, Red Deer and Elk (Wapiti). The last two species were once considered a single species, illustrating how species boundaries may change with increased scientific knowledge.

Because of the difficulties with both defining and tallying the total numbers of different species in the world, it is estimated that there are anywhere between 2 and 100 million different species.

Placement within genera
Each species is placed within a single genus. This is a hypothesis that the species is more closely related to other species within its genus than to species of other genera. All species are given a binomial name consisting of the generic name and specific name (or specific epithet). For example, Pinus palustris (commonly known as the Longleaf Pine). The taxonomic ranks are life, domain, kingdom, phylum, class, order, family, genus, and species.

In scientific classification, a species is assigned a two-part name, treated as Latin, although roots from any language can be used as well as names of locales or individuals. The genus is listed first (with its leading letter capitalized), followed by a second term: for example, gray wolves belong to the species Canis lupus, coyotes to Canis latrans, golden jackals to Canis aureus, etc., and all of those belong to the genus Canis (which also contains many other species). The name of the species is the whole binomial, not just the second term (which may be called specific name for animals).

The binomial naming convention that is used, later formalized in the biological codes of nomenclature, was first used by Leonhart Fuchs and introduced as the standard by Carolus Linnaeus in his 1758 classical work Systema Naturae 10th edition. As a result, it is sometimes called the "binomial nomenclature". At that time, the chief biological theory was that species represented independent acts of creation by God and were therefore considered objectively real and immutable.

Abbreviated names
Books and articles sometimes intentionally do not identify species fully and use the abbreviation "sp." in the singular or "spp." in the plural in place of the specific epithet: for example, Canis sp. This commonly occurs in the following types of situations:


 * The authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong. This is particularly common in paleontology.


 * The authors use "spp." as a short way of saying that something applies to many species within a genus, but do not wish to say that it applies to all species within that genus. If scientists mean that something applies to all species within a genus, they use the genus name without the specific epithet.

In books and articles, genus and species names are usually printed in italics. If using "sp." and "spp.", these should not be italicized.