In biology, a species (/ˈspiːʃiːz/ // (listen)) is the basic
unit of classification and a taxonomic rank of an organism, as well as
a unit of biodiversity. A species is often defined as the largest
group of organisms in which any two individuals of the appropriate
sexes or mating types can produce fertile offspring, typically by
sexual reproduction. Other ways of defining species include their
1.1 Typological or morphological species 1.2 Recognition and cohesion species 1.3 Genetic similarity and barcode species 1.4 Phylogenetic, cladistic, or evolutionary species 1.5 Ecological species 1.6 Genetic species 1.7 Evolutionarily significant unit 1.8 Chronospecies 1.9 Viral quasispecies
2 Taxonomy and naming
2.1 Common and scientific names
3 Mayr's biological species concept 4 The species problem
4.1 When Mayr's concept breaks down 4.2 Aggregates of microspecies 4.3 Hybridisation 4.4 Ring species
5.1 Speciation 5.2 Exchange of genes between species 5.3 Extinction
6 Practical implications 7 History
7.1 Classical forms 7.2 Fixed species 7.3 Mutability
8 See also 9 Notes 10 Cited sources 11 External links
Biologists and taxonomists have made many attempts to define species,
beginning from morphology and moving towards genetics. Early
taxonomists such as Linnaeus had no option but to describe what they
saw: this was later formalised as the typological or morphological
Typological or morphological species
All adult Eurasian blue tits share the same coloration, unmistakably
identifying the morphospecies.
A typological species is a group of organisms in which individuals
conform to certain fixed properties (a type), so that even
pre-literate people often recognise the same taxon as do modern
taxonomists. The clusters of variations or
phenotypes within specimens (such as longer or shorter tails) would
differentiate the species. This method was used as a "classical"
method of determining species, such as with Linnaeus early in
evolutionary theory. However, different phenotypes are not necessarily
different species (e.g. a four-winged
Recognition and cohesion species A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridize successfully, they are still distinct cohesion species if the amount of hybridization is insufficient to completely mix their respective gene pools. A further development of the recognition concept is provided by the biosemiotic concept of species.
Genetic similarity and barcode species
A region of the gene for the cytochrome c oxidase enzyme is used to
distinguish species in the Barcode of
Phylogenetic, cladistic, or evolutionary species 
The cladistic or phylogenetic species concept is that a species is
the smallest lineage which is distinguished by a unique set of either
genetic or morphological traits. No claim is made about reproductive
isolation, making the concept useful also in palaeontology where only
fossil evidence is available.
A phylogenetic or cladistic species is an evolutionarily divergent
lineage, one that has maintained its hereditary integrity through time
and space. A cladistic species is
the smallest group of populations that can be distinguished by a
unique set of morphological or genetic traits. Molecular markers may
be used to determine genetic similarities in the nuclear or
Ecological species An ecological species is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters.
A genetic species as defined by Robert Baker and Robert Bradley is a
set of genetically isolated interbreeding populations. This is similar
to Mayr's Biological
Evolutionarily significant unit An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation.
Chronospecies A chronospecies is defined in a single lineage (solid line) whose morphology changes with time. At some point, palaeontologists judge that enough change has occurred that two species (A and B), separated in time and anatomy, once existed. Main article: Chronospecies In palaeontology, with only comparative anatomy (morphology) from fossils as evidence, the concept of a chronospecies can be applied. During anagenesis (evolution, not necessarily involving branching), palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change. In such a time sequence, palaeontologists assess how much change is required for a morphologically distinct form to be considered a different species from its ancestors.
Main article: Viral quasispecies
Taxonomy and naming
A cougar, mountain lion, panther, or puma, among other common names:
its scientific name is Puma concolor.
Common and scientific names
The commonly used names for kinds of organisms are often ambiguous:
"cat" could mean the domestic cat, Felis catus, or the cat family,
Felidae. Another problem with common names is that they often vary
from place to place, so that puma, cougar, catamount, panther, painter
and mountain lion all mean
Abbreviations Books and articles sometimes intentionally do not identify species fully and use the abbreviation "sp." in the singular or "spp." (standing for species pluralis, the Latin for multiple species) in the plural in place of the specific name or epithet (e.g. Canis sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology. Authors may also use "spp." as a short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics. Abbreviations such as "sp." should not be italicised. When a species identity is not clear a specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after.
Identification codes With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including:
National Center for Biotechnology Information
Broad and narrow senses
Main article: Sensu
The nomenclatural codes that guide the naming of species, including
the ICZN for animals and the ICN for plants, do not make rules for
defining the boundaries of the species. Research can change the
boundaries, also known as circumscription, based on new evidence.
Mayr's biological species concept
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It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in the wild.
The species problem
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.
When Mayr's concept breaks down
Palaeontologists are limited to morphological evidence when deciding
whether fossil life-forms like these
When organisms reproduce asexually, as in single-celled organisms such
as bacteria and other prokaryotes, and parthenogenetic or
apomictic multi-celled organisms.
The term quasispecies is sometimes used for rapidly mutating entities
When scientists do not know whether two morphologically similar groups
of organisms are capable of interbreeding; this is the case with all
extinct life-forms in palaeontology, as breeding experiments are not
When hybridisation permits substantial gene flow between
In ring species, when members of adjacent populations in a widely
continuous distribution range interbreed successfully but members of
more distant populations do not.
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.mw-parser-output .tmulti .thumbcaption text-align:center The willow
warbler and chiffchaff are almost identical in appearance but do not
there is no easy way to tell whether related geographic or temporal
forms belong to the same or different species.
Aggregates of microspecies
Blackberries belong to any of hundreds of microspecies of the Rubus fruticosus species aggregate.
The butterfly genus
Hybridisation Main article: Hybrid (biology) Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they hybridise freely where their geographical ranges overlap.
Hybridisation of carrion and hooded crows permits gene flow between 'species'
Hybrid with dark belly, dark gray nape
Hybrid with dark belly
Main article: Ring species
A ring species is a connected series of neighbouring populations, each
of which can sexually interbreed with adjacent related populations,
but for which there exist at least two "end" populations in the
series, which are too distantly related to interbreed, though there is
a potential gene flow between each "linked" population.
Such non-breeding, though genetically connected, "end" populations may
co-exist in the same region thus closing the ring.
Seven "species" of
A greenish warbler, Phylloscopus trochiloides
Presumed evolution of five "species" of greenish warblers around Himalayas
Main article: Speciation
The evolutionary process by which biological populations evolve to
become distinct or reproductively isolated as species is called
Exchange of genes between species
Horizontal gene transfers between widely separated species
complicate the phylogeny of bacteria.
Main article: Horizontal gene transfer
Horizontal gene transfer
Practical implications Biologists and conservationists need to categorise and identify organisms in the course of their work. Difficulty assigning organisms reliably to a species constitutes a threat to the validity of research results, for example making measurements of how abundant a species is in an ecosystem moot. Surveys using a phylogenetic species concept reported 48% more species and accordingly smaller populations and ranges than those using nonphylogenetic concepts; this was termed "taxonomic inflation", which could cause a false appearance of change to the number of endangered species and consequent political and practical difficulties. Some observers claim that there is an inherent conflict between the desire to understand the processes of speciation and the need to identify and to categorise. Conservation laws in many countries make special provisions to prevent species from going extinct. Hybridization zones between two species, one that is protected and one that is not, have sometimes led to conflicts between lawmakers, land owners and conservationists. One of the classic cases in North America is that of the protected northern spotted owl which hybridizes with the unprotected California spotted owl and the barred owl; this has led to legal debates. It has been argued that the species problem is created by the varied uses of the concept of species, and that the solution is to abandon it and all other taxonomic ranks, and use unranked monophyletic groups instead. It has been argued, too, that since species are not comparable, counting them is not a valid measure of biodiversity; alternative measures of phylogenetic biodiversity have been proposed.
Main article: Aristotle's biology
In his biology,
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.
In viewing evidence of hybridisation, Linnaeus recognised that species
were not fixed and could change; he did not consider that new species
could emerge and maintained a view of divinely fixed species that may
alter through processes of hybridisation or
acclimatisation. 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
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.
Lists of animal species Category:Lists of animal species Category:Lists of plant species Cline Encyclopedia of Life Endangered species Global biodiversity Systematics
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Realm (vir.) Subrealm (vir.) Domain/Superkingdom Kingdom Subkingdom Infrakingdom/Branch
Superphylum/Superdivision Phylum/Division Subphylum Infraphylum Microphylum
Superclass Class Subclass Infraclass Parvclass
Magnorder Superorder Order Suborder Infraorder Parvorder
Section (zoo.) Superfamily Family Subfamily Infrafamily
Supertribe Tribe Subtribe Infratribe
Genus Subgenus Section (bot.) Series (bot.)
Species Subspecies Variety Form