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

Other articles related to "species":

Organic Farming - Externalities - Biodiversity
... Nearly all non-crop, naturally occurring species observed in comparative farm land practice studies show a preference for organic farming both by abundance and diversity ... An average of 30% more species inhabit organic farms ... Many weed species attract beneficial insects that improve soil qualities and forage on weed pests ...
... All species have a proboscis which lies in the rhynchocoel when inactive but everts (turns inside-out) to emerge just above the mouth and capture the animal ... A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host ... on a trail of slime, while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations ...
Nemertea - Taxonomy
... Comprises 100 marine species ... Comprises about 400 species ... Includes seven species, of which six live as commensals in the mantle of large clams and one in that of a freshwater snail ...
Orthoptera - Life Cycle
... Orthopteroid species have a paurometabolous life cycle or incomplete metamorphosis ... of sound is generally crucial in courtship, and most species have distinct songs ... The number of moults varies between species growth is also very variable and may take a few weeks to some months depending on food availability and weather conditions ...
Nemertea - Description - Nervous System and Senses
... Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord ... In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers ... Some species have paired cerebral organs, sacs whose only openings are to the outside ...

Famous quotes containing the word species:

    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)

    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)

    Prostitution is the most hideous of the afflictions produced by the unequal distribution of the world’s goods; this infamy stigmatizes the human species and bears witness against the social organization far more than does crime.
    Flora Tristan (1803–1844)