Species

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LifeDomainKingdomPhylumClassOrderFamilyGenusSpecies
The hierarchy of biological classification's eight major taxonomic ranks. A genus contains one or more species. Intermediate minor rankings are not shown.

In

paleontologists use the concept of the chronospecies since fossil
reproduction cannot be examined.

The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million.[1][2][3] However, only about 14% of these had been described by 2011.[3]

All species (except

zoological nomenclature). For example, Boa constrictor is one of the species of the genus Boa
, with constrictor being the species' epithet.

While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic

microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change. This obliges taxonomists to decide, for example, when enough change has occurred to declare that a lineage should be divided into multiple chronospecies, or when populations have diverged to have enough distinct character states to be described as cladistic
species.

Species were seen from the time of

Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies
.

Definition

Biologists and taxonomists have made many attempts to define species, beginning from

ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species, a species as determined by a taxonomist.[8]

Typological or morphological species

All adult Eurasian blue tits share the same coloration, unmistakably identifying the morphospecies.[9]

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.[10][11] 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]

Recognition and cohesion species

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]

Genetic similarity and barcode species

Barcode of Life Data Systems
database.

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

average nucleotide identity method quantifies genetic distance between entire genomes, using regions of about 10,000 base pairs. With enough data from genomes of one genus, algorithms can be used to categorize species, as for Pseudomonas avellanae in 2013,[22] and for all sequenced bacteria and archaea since 2020.[23]

Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species.[28]

Phylogenetic or cladistic species

A phylogenetic or

red list and can attract conservation legislation and funding.[40]

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.[29] Therefore, it applies to asexual lineages.[34][35] 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.[35]

Evolutionary species

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][41] 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.[42] A version of the concept is Kevin de Queiroz's "General Lineage Concept of Species".[43]

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.[44]

Genetic species

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.

allozymes (enzyme variants).[46]

Evolutionarily significant unit

An evolutionarily significant unit (ESU) or "wildlife species"[47] is a population of organisms considered distinct for purposes of conservation.[48]

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.[49][50][51][52]

Viral quasispecies

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.

mutagenic environment, and hence governed by a mutation–selection balance. It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles a traditional biological species.[54][55][56] The International Committee on Taxonomy of Viruses has since 1962 developed a universal taxonomic scheme for viruses; this has stabilised viral taxonomy.[57][58][59]

Mayr's biological species concept

Biological Species Concept
of reproductive isolation in 1942.

Most modern textbooks make use of

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[62]

groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.[62]

It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.[63][64][65][66] 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.[62]

The species problem

It is difficult to define a species in a way that applies to all organisms.[67] The debate about species concepts is called the species problem.[62][68][69][70] The problem was recognised even in 1859, when Darwin wrote in 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.[71]

When Mayr's concept breaks down

Palaeontologists are limited to morphological evidence when deciding whether fossil life-forms like these Inoceramus
bivalves formed a separate species.

A simple textbook definition, following Mayr's concept, works well for most

multi-celled organisms
, but breaks down in several situations:

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.

cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).[81] In addition, horizontal gene transfer (HGT) makes it difficult to define a species.[82] All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT.[83] There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes,[82] including some crustaceans and echinoderms.[84]

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]

Wilkins writes that biologists such as the botanist Brent Mishler[85] have argued that the species concept is not valid, and that "if we were being true to evolution and the consequent phylogenetic approach to taxa, we should replace it with a 'smallest clade' idea" (a phylogenetic species concept).[86] Wilkins states that he concurs[87] with this approach, while noting the difficulties it would cause to taxonomy. He cites the ichthyologist Charles Tate Regan's early 20th century remark that "a species is whatever a suitably qualified biologist chooses to call a species".[86] Wilkins notes that the philosopher Philip Kitcher called this the "cynical species concept",[88] and arguing that far from being cynical, it usefully leads to an empirical taxonomy for any given group, based on taxonomists' experience.[86]

Aggregates of microspecies

The species concept is further weakened by the existence of

fly agaric.[96]

Hybridisation

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 can hybridise where their geographical ranges overlap.[97]

Ring species

A

Ensatina eschscholtzii group of 19 populations of salamanders in America,[100] and the greenish warbler in Asia,[101] but many so-called ring species have turned out to be the result of misclassification leading to questions on whether there really are any ring species.[102][103][104][105]

Taxonomy and naming

Puma concolor
.