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:
“Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man’s appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded.”
—Abraham Lincoln (1809–1865)
“Genius detects through the fly, through the caterpillar, through the grub, through the egg, the constant individual; through countless individuals the fixed species; through many species the genus; through all genera the steadfast type; through all the kingdoms of organized life the eternal unity. Nature is a mutable cloud which is always and never the same.”
—Ralph Waldo Emerson (1803–1882)
“Nature seemed to have adorned herself for our departure with a profusion of fringes and curls, mingled with the bright tints of flowers, reflected in the water. But we missed the white water-lily, which is the queen of river flowers, its reign being over for this season.... Many of this species inhabit our Concord water.”
—Henry David Thoreau (1817–1862)