Life Domain Kingdom Phylum Class Order Family Genus Species
The hierarchy of biological classification's eight major taxonomic ranks. A genus contains one or more species. Intermediate minor rankings are not shown.

In biology, a species is the basic unit of classification and a taxonomic rank, as well as a unit of biodiversity, but it has proven difficult to find a satisfactory definition. Scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If as Linnaeus thought, species were fixed, there would be no problem, but evolutionary processes cause species to change continually, and to grade into one another. A species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction. While this definition is often adequate, when looked at more closely it is problematic. For example, with hybridisation, in a species complex of hundreds of similar microspecies, or in a ring species, the boundaries between closely related species become unclear. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Problems also arise when dealing with fossils, since reproduction cannot be examined; the concept of the chronospecies is therefore used in palaeontology. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche.

All species are given a two-part name, a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). For example, Boa constrictor is one of four species of the Boa genus.

Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection. That understanding was greatly extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures. Genes can sometimes be exchanged between species by horizontal gene transfer; new species can arise rapidly through hybridisation and polyploidy; and species may become extinct for a variety of reasons. Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies.

As a practical matter, species concepts may be used to define species that are then used to measure biodiversity, though whether this is a good measure is disputed, as other measures are possible.


Classical forms

In his biology, Aristotle used the term γένος (génos) to mean a kind, such as a bird or fish, and εἶδος (eidos) to mean a specific form within a kind, such as (within the birds) the crane, eagle, crow, or sparrow. These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named; today the birds are a class, the cranes are a family, and the crows a genus. A kind was distinguished by its attributes; for instance, a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood. A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents. Aristotle believed all kinds and forms to be distinct and unchanging. His approach remained influential until the Renaissance. [1]

Fixed species

John Ray believed that species breed true and do not change, even though variations exist.

When observers in the Early Modern period began to develop systems of organization for living things, they placed each kind of animal or plant into a context. Many of these early delineation schemes would now be considered whimsical: schemes included consanguinity based on colour (all plants with yellow flowers) or behaviour (snakes, scorpions and certain biting ants). John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, as follows:

No surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species ... Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa. [2]

Carl Linnaeus created the binomial system for naming species.

In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. [3] He established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships. [4] [5] At the time, however, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. This view was influenced by European scholarly and religious education, which held that the categories of life are dictated by God, forming an Aristotelian hierarchy, the scala naturae or great chain of being. However, whether or not it was supposed to be fixed, the scala (a ladder) inherently implied the possibility of climbing. [6]

The possibility of change

Faced with evidence of hybridisation, Linnaeus came to accept that species could change, and the struggle for survival, but not that new species could freely evolve. [7] By the 19th century, naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. Jean-Baptiste Lamarck, in his 1809 Zoological Philosophy, described the transmutation of species, proposing that a species could change over time, in a radical departure from Aristotelian thinking. [8]

In 1859, Charles Darwin and Alfred Russel Wallace provided a compelling account of evolution and the formation of new species. Darwin argued that it was populations that evolved, not individuals, by natural selection from naturally occurring variation among individuals. [9] This required a new definition of species. Darwin concluded that species are what they appear to be: ideas, provisionally useful for naming groups of interacting individuals, writing:

I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other ... It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake. [10]

Other Languages
Afrikaans: Spesie
Alemannisch: Art (Biologie)
العربية: نوع (تصنيف)
aragonés: Especie
asturianu: Especie
Avañe'ẽ: Juehegua
azərbaycanca: Bioloji növ
বাংলা: প্রজাতি
башҡортса: Төр (биология)
беларуская: Біялагічны від
беларуская (тарашкевіца)‎: Від (біялёгія)
भोजपुरी: प्रजाति
български: Вид (биология)
Boarisch: Oart (Biologie)
brezhoneg: Spesad
català: Espècie
Чӑвашла: Тĕс (биологи)
čeština: Druh
Cymraeg: Rhywogaeth
dansk: Art
español: Especie
Esperanto: Specio
euskara: Espezie
Fiji Hindi: Species
français: Espèce
Frysk: Soarte
Gaeilge: Speiceas
galego: Especie
한국어: 종 (생물학)
hornjoserbsce: Družina (biologija)
hrvatski: Vrsta
Ilokano: Sebbangan
Bahasa Indonesia: Spesies
interlingua: Specie
italiano: Specie
Basa Jawa: Spésies
ಕನ್ನಡ: ಜಾತಿ
Kapampangan: Species
къарачай-малкъар: Биология тюрлю
ქართული: სახეობა
қазақша: Түр
Kiswahili: Spishi
Kreyòl ayisyen: Espès
Kurdî: Cure
Кыргызча: Түр
latviešu: Suga
Lëtzebuergesch: Aart
lietuvių: Rūšis
lumbaart: Spéce
magyar: Faj
македонски: Вид (биологија)
മലയാളം: സ്പീഷീസ്
Malti: Speċi
მარგალური: გვარობა
Bahasa Melayu: Spesies
монгол: Зүйл
မြန်မာဘာသာ: မျိုးစိတ်
Nederlands: Soort
नेपाल भाषा: प्रजाति
日本語: 種 (分類学)
Napulitano: Specia
norsk: Art
norsk nynorsk: Art
oʻzbekcha/ўзбекча: Tur (biologiya)
ਪੰਜਾਬੀ: ਪ੍ਰਜਾਤੀ
پنجابی: سپیشیز
Patois: Spiishi
Piemontèis: Spece
Plattdüütsch: Oort (Biologie)
português: Espécie
Runa Simi: Rikch'aq
русиньскый: Вид (біолоґія)
Scots: Species
shqip: Lloji
Simple English: Species
slovenčina: Druh (taxonómia)
slovenščina: Vrsta (biologija)
کوردی: جۆرە
српски / srpski: Врста (биологија)
srpskohrvatski / српскохрватски: Vrsta
Basa Sunda: Spésiés
suomi: Laji
svenska: Art
Tagalog: Espesye
татарча/tatarça: Төр (биология)
తెలుగు: జాతి
Türkçe: Tür
Türkmençe: Biologik görnüş
українська: Вид
اردو: نوع
vèneto: Spece
Tiếng Việt: Loài
West-Vlams: Sôorte
Winaray: Espesyis
ייִדיש: זגאל
粵語: 物種
中文: 物种