Species
In
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
While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic
Species were seen from the time of
Definition
Biologists and taxonomists have made many attempts to define species, beginning from
Typological or morphological species
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
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
Phylogenetic or cladistic 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
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.
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
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.In
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.
Mayr's biological species concept
Most modern textbooks make use of
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
A simple textbook definition, following Mayr's concept, works well for most
- When organisms parthenogenetic or apomictic multi-celled organisms. DNA barcoding and phylogenetics are commonly used in these cases.[73][74][75] The term quasispecies is sometimes used for rapidly mutating entities like viruses.[76][77]
- 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.[78]
- When hybridisation permits substantial gene flow between species.[79]
- In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not.[80]
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]
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
Blackberries belong to any of hundreds of microspecies of the
species aggregate.
The butterfly genus Heliconius contains many similar species.
The
fasciatusspecies complex contains at least six species of treefrog.
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]
- Hybridisation of carrion and hooded crows permits gene flow between 'species'
Hybrid with dark belly
Ring species
A
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
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