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 the largest 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, and where available, their DNA sequences. All species are given a two-part name, a “binomial name”, or just “binomial”. The first part of a binomial is the generic name, the genus to which the species belongs. 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 binomial is capitalized, and the second part has a lower case. The binomial 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.[1] 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.[1][2]

The total number of non-bacterial and non-archaeal species in the world has been estimated at 8.7 million,[3][4] with previous estimates ranging from two million to 100 million.[5]

Biologists’ working definition

A usable definition of the word “species” and reliable methods of identifying particular species is 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. It is generally difficult to give precise taxonomic rankings to extinct species known only from fossils.

Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences 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 of concern.

Difficulty of defining “species” and identifying particular species

Main article: Species problem

It is surprisingly difficult to define the word “species” in a way that applies to all naturally occurring organisms, and the debate among biologists about how to define “species” and how to identify actual species is called the species problem. Over two dozen distinct definitions of “species” are in use amongst biologists.

Most textbooks follow Ernst Mayr’s definition, known as the Biological Species Concept (BSC) of a species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups”. It has been argued that this definition of species is not only a useful formulation, but is also a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.

Various parts of this definition serve to exclude some unusual or artificial matings:

Those that occur only in captivity (when the animal’s normal mating partners may not be available) or as a result of deliberate human action
Animals that may be physically and physiologically capable of mating but, for various reasons, do not normally do so in the wild
The typical textbook definition above works well for most multi-celled organisms, but there are several types of situations in which it breaks down:

By definition it applies only to organisms that reproduce sexually. So it does not work for asexually reproducing single-celled organisms and for the relatively few parthenogenetic or apomictic multi-celled organisms. The term “phylotype” is often applied to such organisms.
Biologists frequently do not know whether two morphologically similar groups of organisms are “potentially” capable of interbreeding.
There is considerable variation in the degree to which hybridization may succeed under natural conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed.
In ring species, members of adjacent populations interbreed successfully but members of some non-adjacent populations do not.
In a few cases it may be physically impossible for animals that are members of the same species to mate. However, these are cases, such as in breeds of dogs, in which human intervention has caused gross morphological changes, and are therefore excluded by the biological species concept.[dubious – discuss]
Horizontal gene transfer makes it even more difficult to define the word “species”. There is strong evidence of horizontal gene transfer between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes; and Williamson argues that there is evidence for it in some crustaceans and echinoderms. All definitions of the word “species” assume that an organism gets all its genes from one or two parents that are very like that organism, but horizontal gene transfer makes that assumption false.

Charles Darwin wrote in chapter II of On the Origin of Species:

No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.

But later, in The Descent of Man, when addressing “The question whether mankind consists of one or several species”, Darwin revised his opinion to say:

it is a hopeless endeavour to decide this point on sound grounds, until some definition of the term “species” is generally accepted; and the definition must not include an element that cannot possibly be ascertained, such as an act of creation.