Species

A species (pl. species) is often defined as the largest group of

A typological species is a group of organisms in which individuals conform to certain fixed properties (a type, which may be defined by a chosen 'nominal species'), so that even pre-literate people often recognise the same taxon as do modern taxonomists.^[10][11] Modern-day field guides and identification websites such as iNaturalist use this concept. 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 Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies.^[12][13]

In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on the morphological species concept, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms.^[14] It differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits.^[15]

A mate-recognition species is a group of sexually reproducing organisms that recognise one another as potential mates.^[16][17] 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 hybridise successfully, they are still distinct cohesion species if the amount of hybridisation is insufficient to completely mix their respective gene pools.^[18] A further development of the recognition concept is provided by the biosemiotic concept of species.^[19]

In microbiology, genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA–DNA hybridisation to decide if they belong to the same species.^[20] This concept was narrowed in 2006 to a similarity of 98.7%.^[21]

The

A phylogenetic or

Unlike the biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts.[30] Therefore, it applies to asexual lineages.^[35][36] However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results.^[36]

An evolutionary species, suggested by George Gaylord Simpson in 1951, is "an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies".^[7][42] This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to Willi Hennig's species-as-lineages concept, and asserted that the biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species.^[43] A version of the concept is Kevin de Queiroz's "General Lineage Concept of Species".^[44]

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.^[45]

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 Species Concept, but stresses genetic rather than reproductive isolation.

An evolutionarily significant unit (ESU) or "wildlife species"^[48] is a population of organisms considered distinct for purposes of conservation.^[49]

In

Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly. All of these factors make conventional species concepts largely inapplicable.

Most modern textbooks make use of Ernst Mayr's 1942 definition,^[62][63] known as the biological species concept, as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines a species as^[64]

groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.^[64]

It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.^{[65][66][67][68]} 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.^[64]

It is difficult to define a species in a way that applies to all organisms.^[69] The debate about species concepts is called the species problem.^{[64][70][71][72]} The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species:

I was much struck how entirely vague and arbitrary is the distinction between species and varieties.^[73]

He went on to write:

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.^[74]

• When organisms
  parthenogenetic or apomictic multi-celled organisms. DNA barcoding and phylogenetics are commonly used in these cases.^[76][77][78] The term quasispecies is sometimes used for rapidly mutating entities like viruses.^[79][80]
• 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 possible.^[81]
• When hybridisation permits substantial gene flow between species.^[82]
• In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not.^[83]

The willow warbler and chiffchaff are almost identical in appearance but do not interbreed.

Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g.

The evolutionary biologist James Mallet concludes that

there is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods.^[18]

The botanist

The species concept is further weakened by the existence of

• 
  Blackberries belong to any of hundreds of microspecies of the

  species aggregate

  .
• 
  The butterfly genus Heliconius contains many similar species.
• 
  The

  fasciatus

  species complex contains at least six species of treefrog.

• 
  Seven "species" of Larus gulls interbreed in a ring around the Arctic.
• 
  Opposite ends of the ring: a herring gull (

  Larus fuscus

  ) in Norway
• 
  A greenish warbler, Phylloscopus trochiloides
• 
  Presumed

  Himalayas

The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat,

A species is given a taxonomic name when a type specimen is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, which may not be based solely on morphology^[115] (see cryptic species), differentiating it from other previously described and related or confusable species and provides a validly published name (in botany) or an available name (in zoology) when the paper is accepted for publication. The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens.^{[116][117][118]} Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature, are "appropriate, compact, euphonious, memorable, and do not cause offence".^[119]

Books and articles sometimes intentionally do not identify species fully, using the abbreviation "sp." in the singular or "spp." (standing for species pluralis, 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.^[120]

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

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.[120]

With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including:

• Homo sapiens is 9606.^[121]
• Kyoto Encyclopedia of Genes and Genomes (KEGG) employs a three- or four-letter code for a limited number of organisms; in this code, for example, H. sapiens is simply hsa.^[122]
• UniProt employs an "organism mnemonic" of not more than five alphanumeric characters, e.g., HUMAN for H. sapiens.^[123]
• Integrated Taxonomic Information System (ITIS) provides a unique number for each species. The LSID for Homo sapiens is urn:lsid:catalogueoflife.org:taxon:4da6736d-d35f-11e6-9d3f-bc764e092680:col20170225.^[124]

The naming of a particular species, including which genus (and higher taxa) it is placed in, is a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two species names are discovered to apply to the same species, the older species name is given

The

Species are subject to change, whether by evolving into new species,[130] exchanging genes with other species,^[131] merging with other species or by becoming extinct.^[132]

The

Horizontal gene transfer between organisms of different species, either through hybridisation, antigenic shift, or reassortment, is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic domains, making analysis of their relationships difficult, and weakening the concept of a bacterial species.^{[137][85][138][131]}

Louis-Marie Bobay and Howard Ochman suggest, based on analysis of the genomes of many types of bacteria, that they can often be grouped "into communities that regularly swap genes", in much the same way that plants and animals can be grouped into reproductively isolated breeding populations. Bacteria may thus form species, analogous to Mayr's biological species concept, consisting of asexually reproducing populations that exchange genes by homologous recombination.^[139][140]

A species is extinct when the

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",^[144] which could cause a false appearance of change to the number of endangered species and consequent political and practical difficulties.^[145][146] 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.^[146]

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

It has been argued, that since species are not comparable, simply counting them is not a valid measure of biodiversity; alternative measures of phylogenetic biodiversity have been proposed.^{[148][89][149]}

In

When observers in the

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.[154]

In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences.^[155] Like many contemporary systematists,^{[156][157][158]} he established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships.^[159][160] At the time, however, it was still widely believed that there was no organic connection between species (except, possibly, between those of a given genus),^[93] no matter how similar they appeared. This view was influenced by European scholarly and religious education, which held that the taxa had been created 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.^[161]

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.^[162] 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.^[163]

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.^[164] 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.^[165]

Sources

• Claridge, M. F.; Dawah, H. A.; Wilson, M. R., eds. (1997). Species: The Units of Biodiversity.
  ISBN 978-0-412-63120-7
  .
• ISBN 978-0-231-10143-1
  .
• Zachos, Frank E. (2016). Species Concepts in Biology: Historical Development, Theoretical Foundations and Practical Relevance.
  ISBN 978-3-319-44964-7
  .

External links

Wikiquote has quotations related to Species.

• Wikispecies – The free species directory that anyone can edit from the Wikimedia Foundation
• Barcoding of species
• Catalogue of Life
• European Species Names in Linnaean, Czech, English, German and French
• "Species" entry at the
  Stanford Encyclopedia
  of Philosophy
• VisualTaxa