In biology, a species (plural: 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, the difficulty of defining species is known as the species problem. Differing measures are often used, such as similarity of DNA, morphology, or ecological niche. Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae).
Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, especially their DNA sequences, where available. All species are given a two-part name, a "binomial name". The first part of a binomial name is the generic name, the genus of the species. The second part is either called the specific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology). For example, Boa constrictor is one of four species of the Boa genus. The first part of the name is capitalized, and the second part has a lower case. The binomial name is written in italics when printed and underlined when handwritten.
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, though other taxonomic levels such as families may be considered in broad-scale studies. Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.
The total number of non-bacterial and non-archaeal species in the world has been estimated at 8.7 million, with previous estimates ranging from two million to 100 million.
Read more about Species: Biologists' Working Definition, Difficulty of Defining "species" and Identifying Particular Species, Definitions of Species, Numbers of Species, Importance in Biological Classification, Implications of Assignment of Species Status, Historical Development of The Species Concept, See Also
Famous quotes containing the word species:
“If there is a species which is more maltreated than children, then it must be their toys, which they handle in an incredibly off-hand manner.... Toys are thus the end point in that long chain in which all the conditions of despotic high-handedness are in play which enchain beings one to another, from one species to another—cruel divinities to their sacrificial victims, from masters to slaves, from adults to children, and from children to their objects.”
—Jean Baudrillard (b. 1929)
“Can it then be doubted, but that God, who is infinitely fine Spirit, and withal intelligent, can make and change all species and kind of body as he pleaseth? But I dare not say, that this is the way by which God Almighty worketh, because it is past my apprehension: yet it serves very well to demonstrate, that the omnipotence of God implieth no contradiction.”
—Thomas Hobbes (1579–1688)
“The principle of avoiding the unnecessary expenditure of energy has enabled the species to survive in a world full of stimuli; but it prevents the survival of the aristocracy.”
—Rebecca West (1892–1983)