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Species
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{{Other uses}}{{pp-move-indef}}{{good article}}{{Biological classification}}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 genus Boa.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.

History

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.BOOK, Leroi, Armand Marie, Armand Marie Leroi, The Lagoon: How Aristotle Invented Science, Aristotle's Lagoon, Bloomsbury, 2014, 978-1-4088-3622-4, 88–90,

Fixed species

File:John Ray from NPG.jpg|upright|thumb|left|John RayJohn RayWhen 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:, quoted in BOOK, Mayr, Ernst, 1982, The growth of biological thought: diversity, evolution, and inheritance, Belknap Press, 256, }}File:Carl von Linné.jpg|upright|thumb|Carl LinnaeusCarl LinnaeusIn the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences.BOOK, P. H., Davis, V. H., Heywood, 1973, Principles of Angiosperm Taxonomy, Huntington, New York, Robert E. Krieger Publishing Company, 17, He established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships.BOOK, James L., Reveal, James S., Pringle, Flora of North America, 1993, 7. Taxonomic Botany and Floristics, Oxford University Press, 0-19-505713-9, 160–161, BOOK, George Gaylord, Simpson, 1961, Principles of Animal Taxonomy, Columbia University Press, 56–57, 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.JOURNAL, Mahoney, Edward P., Lovejoy and the Hierarchy of Being, Journal of the History of Ideas, 48, 2, 211–230, 10.2307/2709555,

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.WEB, Carl Linnaeus (1707–1778),weblink UCMP Berkeley, 24 January 2018, no,weblink" title="web.archive.org/web/20110430160025weblink">weblink 30 April 2011, dmy-all, 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.BOOK, Gould, Stephen Jay, Stephen Jay Gould, The Structure of Evolutionary Theory, 2002, Belknap Harvard, Harvard, 978-0-674-00613-3, 170–197, 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.BOOK, Bowler, Peter J., Peter J. Bowler, 2003, Evolution: The History of an Idea, 3rd, Berkeley, CA, University of California Press, 0-520-23693-9, 177–223 and passim, 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:}}

Taxonomy and naming

File:Mountain-lion-01623.jpg|thumb|A cougar, mountain lion, panther, or puma, among other common names: its scientific name is Puma concolorPuma 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 Puma concolor in various parts of America, while "panther" may also mean the jaguar (Panthera onca) of Latin America or the leopard (Panthera pardus) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together: the genus as in Puma, and the specific epithet as in concolor.WEB, A Word About Species Names ...,weblink Smithsonian Marine Station at Fort Pierce, 11 March 2017, no,weblink" title="web.archive.org/web/20170324161039weblink">weblink 24 March 2017, dmy-all, NEWS, Hone, Dave, What's in a name? Why scientific names are important,weblink 11 March 2017, The Guardian, 19 June 2013, no,weblink 15 February 2017, dmy-all,

Species description

File:Lacerta plica holotype - ZooKeys-355-049-g005.jpg|thumb|upright|The type specimen (holotype) of Lacerta plica, described by LinnaeusLinnaeusA 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, 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.One example of an abstract of an article naming a new species can be found at JOURNAL, 18 June 2011,weblink Methylobacterium cerastii sp. nov., a novel species isolated from the leaf surface of Cerastium holosteoides, S., Wellner, N., Lodders, P., Kämpfer, 2012, International Journal of Systematic and Evolutionary Microbiology, 62, 917–924, 10.1099/ijs.0.030767-0, {{citation |last=Hitchcock |first=A. S. |year=1921 |title=The Type Concept in Systematic Botany |journal=American Journal of Botany |volume=8 |issue=5 |pages=251–255 |jstor=2434993 |doi=10.2307/2434993}}WEB, Nicholson, Dan H., Botanical nomenclature, types, & standard reference works,weblink Smithsonian National Museum of Natural History, Department of Botany, 17 November 2015, no,weblink" title="web.archive.org/web/20150916184608weblink">weblink 16 September 2015, dmy-all, 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".WEB, 18 June 2011,weblink International Code of Zoological Nomenclature, Recommendation 25C, no,weblink" title="web.archive.org/web/20110608184753weblink">weblink 8 June 2011, dmy-all,

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.BOOK, Winston, Judith E., Describing species. Practical taxonomic procedure for biologists, Columbia University Press, New York, 1999, 141–144,

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 (NCBI) employs a numeric 'taxid' or Taxonomy identifier, a "stable unique identifier", e.g., the taxid of Homo sapiens is 9606.WEB,weblink Home – Taxonomy – NCBI, Ncbi.nlm.nih.gov, 19 October 2012, 25 November 2012, no,weblink 7 February 2018, dmy-all,
  • 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.WEB,weblink KEGG Organisms: Complete Genomes, Genome.jp, 25 November 2012, no,weblink" title="web.archive.org/web/20121014160440weblink">weblink 14 October 2012, dmy-all,
  • UniProt employs an "organism mnemonic" of not more than five alphanumeric characters, e.g., HUMAN for H. sapiens.WEB,weblink Taxonomy, Uniprot.org, 25 November 2012, no,weblink" title="web.archive.org/web/20121127091404weblink">weblink 27 November 2012, dmy-all,
  • 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.WEB, ITIS: Homo sapiens,weblink Catalogue of Life, 11 March 2017, no,weblink" title="web.archive.org/web/20170312194754weblink">weblink 12 March 2017, dmy-all,

Lumping and splitting

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 confirmed 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 named species are discovered to be of the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymisation. Dividing a taxon into multiple, often new, taxa is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms.JOURNAL, Simpson, George Gaylord, George Gaylord Simpson, The Principles of Classification and a Classification of Mammals, Bulletin of the American Museum of Natural History, 85, 23, 1945, JOURNAL, Chase, Bob, Upstart Antichrist, History Workshop Journal, 60, 2005, 202–206,

Broad and narrow senses

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. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as the person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies. Other abbreviations such as "auct." ("author") and "non." ("not") may be used to further clarify the sense in which the specified authors delineated or described the species.WEB, Wilson, Philip, sensu stricto, sensu lato,weblink AZ of tree terms, 2016, 9 January 2018, no,weblink" title="web.archive.org/web/20180110054734weblink">weblink 10 January 2018, dmy-all, WEB, Glossary: sensu,weblink International Commission on Zoological Nomenclature, 9 January 2018, no,weblink" title="web.archive.org/web/20170802164704weblink">weblink 2 August 2017, dmy-all, {{anchor|Reproductive species}}{{anchor|Isolation species}}

Mayr's biological species concept

File:Ernst Mayr PLoS.jpg|thumb|Ernst Mayr proposed the widely used Biological Species ConceptBiological Species ConceptMost modern textbooks make use of Ernst Mayr's 1942 definition,BOOK, Mayr, Ernst, Ernst Mayr, 1942, Systematics and the Origin of Species, Columbia University Press, New York, Wheeler, pp. 17–29 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 asJOURNAL, de Queiroz, K., Ernst Mayr and the modern concept of species, PNAS, 102, Supplement 1, 6600–6607, 2005, 15851674, 1131873, 10.1073/pnas.0502030102, {{Open access}}It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.JOURNAL, Hopf, F. A., Hopf, F. W., 1985, The role of the Allee effect on species packing, Theoretical Population Biology, 27, 27–50, 10.1016/0040-5809(85)90014-0, JOURNAL, Bernstein, H., Byerly, H. C., Hopf, F. A., Michod, R. E., Sex and the emergence of species, Journal of Theoretical Biology, 117, 4, 665–690, 1985, 4094459, 10.1016/S0022-5193(85)80246-0, BOOK, Bernstein, Carol, Bernstein, Harris, Aging, sex, and DNA repair, Academic Press, Boston, 1991, 978-0-12-092860-6, BOOK, Michod, Richard E., Eros and Evolution: A Natural Philosophy of Sex, Addison-Wesley, 1995, 0-201-44232-9, 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

It is difficult to define a species in a way that applies to all organisms.{{Citation|title=Fuzzy species revisited |author=Hanage, William P. |journal=BMC Biology |year=2013 |volume=11 |issue=41 |doi=10.1186/1741-7007-11-41 |pmid=23587266 |pmc=3626887 }} The debate about species delimitation is called the species problem.JOURNAL, Koch, H., 2010, Combining morphology and DNA barcoding resolves the taxonomy of Western Malagasy Liotrigona Moure, 1961, African Invertebrates, 51, 2, 413–421, 10.5733/afin.051.0210,weblink no,weblink 1 December 2016, dmy-all, {{Open access}}JOURNAL, De Queiroz K, Species concepts and species delimitation, Syst. Biol., 56, 6, 879–886, 2007, 18027281, 10.1080/10635150701701083, {{Open access}}JOURNAL, Fraser, C., Alm, E.J., Polz, M.F., Spratt, B.G., Hanage, W.P., The bacterial species challenge: making sense of genetic and ecological diversity, Science, 323, 5915, 741–746, 2009, 19197054, 10.1126/science.1159388, {{Closed access}} The problem was recognized even in 1859, when Darwin wrote in On the Origin of Species:}}

When Mayr's concept breaks down

File:Inoceramus cripsii Creta sup Bergamo.JPG|thumb|Palaeontologists are limited to morphological evidence when deciding whether fossil life-forms like these InoceramusInoceramusA simple textbook definition, following Mayr's concept, works well for most multi-celled organisms, but breaks down in several situations:
  • When organisms reproduce asexually, as in single-celled organisms such as bacteria and other prokaryotes,JOURNAL, 10.1038/nrmicro1236, 16138101, Opinion: Re-evaluating prokaryotic species, Nature Reviews Microbiology, 3, 9, 733, 2005, Gevers, Dirk, Cohan, Frederick M., Lawrence, Jeffrey G., Spratt, Brian G., Coenye, Tom, Feil, Edward J., Stackebrandt, Erko, De Peer, Yves Van, Vandamme, Peter, Thompson, Fabiano L., Swings, Jean, and parthenogenetic or apomictic multi-celled organisms.BOOK, Templeton, A. R., 1989, Speciation and its consequences, The meaning of species and speciation: A genetic perspective, Sinauer Associates, D. Otte, J. A. Endler, 3–27, BOOK, Edward G. Reekie, Fakhri A. Bazzaz, Reproductive allocation in plants,weblink 2005, Academic Press, 978-0-12-088386-8, 99, no,weblink" title="web.archive.org/web/20130617224832weblink">weblink 17 June 2013, dmy-all, JOURNAL, Rosselló-Mora, Ramon, Amann, Rudolf, January 2001, The species concept for prokaryotes,weblink FEMS Microbiology Reviews, 25, 1, 39–67, 10.1111/j.1574-6976.2001.tb00571.x, no,weblink 11 January 2018, dmy-all, The term quasispecies is sometimes used for rapidly mutating entities like viruses.JOURNAL, Andino, Raul, Domingo, Esteban, Viral quasispecies,weblink Virology, 479–480, 46–51, 10.1016/j.virol.2015.03.022, no,weblink" title="web.archive.org/web/20180111165149weblink">weblink 11 January 2018, dmy-all, 4826558, BOOK,weblink Quasispecies: Concept and Implications for Virology, Biebricher, C. K., Eigen, M., 2006, Springer, 978-3-540-26397-5, Current Topics in Microbiology and Immunology, 1–31, 10.1007/3-540-26397-7_1,
  • 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.JOURNAL, Teueman, A. E., The Species-Concept in Palaeontology, Geological Magazine, 2009, 61, 8, 355–360, 10.1017/S001675680008660X,weblink no,weblink 14 March 2017, dmy-all,
  • When hybridisation permits substantial gene flow between species.{{sfn|Zachos|2016|p=101}}
  • In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not.{{sfn|Zachos|2016|pp=156–157}}
{{multiple image
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| image1 = Willow Warbler Phylloscopus trochilus.jpg
| alt1 = Willow warbler
| image2 = Common Chiff-Chaff - Italy S4E1681 (19081363189) (cropped).jpg
| alt2 = Chiffchaff
| footer = 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 categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).JOURNAL, 2014, How discordant morphological and molecular evolution among microorganisms can revise our notions of biodiversity on Earth, BioEssays, 36, 10, 950–959, 10.1002/bies.201400056, Lahr, D. J., Laughinghouse, H. D., Oliverio, A. M., Gao, F., Katz, L. A., 25156897, 4288574, In addition, horizontal gene transfer (HGT) makes it difficult to define a species. 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.JOURNAL, Bapteste, E., May 2005, Do orthologous gene phylogenies really support tree-thinking?, BMC Evolutionary Biology, 5, 33, 10.1186/1471-2148-5-33, etal, 15913459, 1156881, There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes,WEB,weblink Melcher, Ulrich, 2001, Molecular genetics: Horizontal gene transfer, Oklahoma State University, no,weblink" title="web.archive.org/web/20160304071146weblink">weblink 4 March 2016, dmy-all, including some crustaceans and echinoderms.BOOK, Williamson, David I., The Origins of Larvae, Kluwer, 2003, 1-4020-1514-3, The evolutionary biologist James Mallet concludes that

Aggregates of microspecies

The species concept is further weakened by the existence of microspecies, groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates.BOOK, Heywood, V. H., 1962, Symposium on Biosystematics, Montreal, October 1962, The "species aggregate" in theory and practice, Heywood, V. H., Löve, Á., 26–36, For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion,BOOK, Pimentel, David, Biological Invasions: Economic and Environmental Costs of Alien Plant, Animal, and Microbe Species,weblink 2014, CRC Press, 978-1-4200-4166-8, 92, no,weblink 7 February 2018, dmy-all, complicated by hybridisation, apomixis and polyploidy, making gene flow between populations difficult to determine, and their taxonomy debatable.JOURNAL, Jarvis, C.E., 1992, Seventy-Two Proposals for the Conservation of Types of Selected Linnaean Generic Names, the Report of Subcommittee 3C on the Lectotypification of Linnaean Generic Names, Taxon, 41, 3, 552–583, 1222833, 10.2307/1222833, JOURNAL, Wittzell, Hakan, 1999, Chloroplast DNA variation and reticulate evolution in sexual and apomictic sections of dandelions, Molecular Ecology, 8, 2023–2035, 10.1046/j.1365-294x.1999.00807.x, 10632854, 12, JOURNAL, Dijk, Peter J. van, 2003, Ecological and evolutionary opportunities of apomixis: insights from Taraxacum and Chondrilla, Philosophical Transactions of the Royal Society B, 358, 1113–1121, 10.1098/rstb.2003.1302, 12831477, 1434, 1693208, Species complexes occur in insects such as Heliconius butterflies,JOURNAL, Mallet, J., Beltrán, M., Neukirchen, W., Linares, M., Natural hybridization in heliconiine butterflies: the species boundary as a continuum, 2007, BMC Evolutionary Biology, 7, 1, 28, 10.1186/1471-2148-7-28, 17319954, 1821009, {{open access}} vertebrates such as Hypsiboas treefrogs,JOURNAL, Ron, Santiago, Caminer, Marcel, Systematics of treefrogs of the Hypsiboas calcaratus and Hypsiboas fasciatus species complex (Anura, Hylidae) with the description of four new species, ZooKeys, 370, 2014, 1–68, 10.3897/zookeys.370.6291, 24478591, 3904076, and fungi such as the fly agaric.JOURNAL, Geml, J., Tulloss, R. E., Laursen, G. A., Sasanova, N. A., Taylor, D. L., 2008, Evidence for strong inter- and intracontinental phylogeographic structure in Amanita muscaria, a wind-dispersed ectomycorrhizal basidiomycete, Molecular Phylogenetics and Evolution, 48, 2, 694–701, 10.1016/j.ympev.2008.04.029, 18547823, File:Ripe, ripening, and green blackberries.jpg|Blackberries belong to any of hundreds of microspecies of the Rubus fruticosus species aggregate.File:Heliconius mimicry.png|The butterfly genus Heliconius contains many similar species.File:Systematics-of-treefrogs-of-the-Hypsiboas-calcaratus-and-Hypsiboas-fasciatus-species-complex-(Anura-ZooKeys-370-001-g009.jpg|The Hypsiboas calcaratus–fasciatus species 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 hybridise freely where their geographical ranges overlap.WEB, Defining a species,weblink University of California Berkeley, 12 March 2017, no,weblink" title="web.archive.org/web/20170313042614weblink">weblink 13 March 2017, dmy-all, File:Krähe_65(loz) (cropped).JPG|Carrion crowFile:20151221Corvus corone.jpg|Hybrid with dark belly, dark gray napeFile:Corvus cornix (Scops).jpg|Hybrid with dark bellyFile:Wrona_Siwa.jpg|Hooded crow

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.{{sfn|Zachos|2016|p=188}} Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing the ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation.BOOK, Stamos, David N., The Species Problem: Biological Species, Ontology, and the Metaphysics of Biology,weblink 2003, Lexington Books, 978-0-7391-6118-0, 330, no,weblink 12 March 2017, dmy-all, However, ring species are at best rare. Proposed examples include the herring gull-lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America,JOURNAL, 10.1093/sysbio/41.3.273, Evolutionary Relationships Within the Ensatina Eschscholtzii Complex Confirm the Ring Species Interpretation, Systematic Biology, 41, 3, 273, 1992, Moritz, C., Schneider, C. J., Wake, D. B.,weblink no,weblink" title="web.archive.org/web/20180112160100weblink">weblink 12 January 2018, dmy-all, and the greenish warbler in Asia,JOURNAL, 10.1126/science.1105201, 15662011, Speciation by Distance in a Ring Species, Science, 307, 5708, 414, 2005, Irwin, D. E., Bensch, Staffan, Irwin, Jessica H., Price, Trevor D., 2005Sci...307..414I, 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.JOURNAL, 10.1016/j.jcz.2007.07.004, Ring species – Do they exist in birds?, Zoologischer Anzeiger - A Journal of Comparative Zoology, 246, 4, 315, 2007, Martens, Jochen, Päckert, Martin, JOURNAL, Alcaide, M., Scordato, E. S. C., Price, T. D., Irwin, D. E., 2014, Genomic divergence in a ring species complex, Nature, 511, 7507, 83–85, 10.1038/nature13285, 24870239, 2014Natur.511...83A, JOURNAL, Liebers, Dorit, Knijff, Peter de, Helbig, Andreas J., The herring gull complex is not a ring species, Proc Biol Sci, 2004, 271, 1542, 893–901, 10.1098/rspb.2004.2679, 1691675, 15255043, JOURNAL, Highton, R., 1998, Is Ensatina eschscholtzii a ring species?, Herpetologica, 54, 254–278, 3893431, File:Ring species seagull.svg|Seven "species" of Larus gulls interbreed in a ring around the Arctic.File:PT05 ubt.jpeg|Opposite ends of the ring: a herring gull (Larus argentatus) (front) and a lesser black-backed gull (Larus fuscus)File:Greenish Warbler Sikkim India 11.05.2014.jpg|A greenish warbler, Phylloscopus trochiloidesFile:Greenish warbler ring.svg|Presumed evolution of five "species" of greenish warblers around Himalayas

Attempts at definition

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 species concept. Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or even impossible to test.JOURNAL, Mallet, James, A species definition for the modern synthesis, Trends in Ecology & Evolution, 1995, 10, 294–299, 10.1016/0169-5347(95)90031-4, Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others.JOURNAL, Masters, J. C., Spencer, H. G., Why We Need a New Genetic Species Concept, Systematic Zoology, 1989, 38, 3, 270–279, 10.2307/2992287, 2992287, Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts,BOOK, Mayden, R. L., M. F. Claridge, H. A. Dawah, M. R. Wilson, A hierarchy of species concepts: the denouement of the species problem, The Units of Biodiversity – Species in Practice Special Volume 54, 1997, Systematics Association, and the philosopher of science John Wilkins counted 26.WEB, Species Concepts,weblink Scientific American, 14 March 2017, 20 April 2012, no,weblink 14 March 2017, dmy-all, Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) 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.{{sfn|Zachos|2016|p=79}}{{anchor|Typological species}}

Typological or morphological species

File:Eurasian_blue_tit_Lancashire.jpg|thumb|All adult Eurasian blue titEurasian blue titA 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.BOOK, Gould, Stephen Jay, Stephen Jay Gould, A Quahog is a Quahog, In: The Panda's Thumb: More Reflections in Natural History, New York, W.W. Norton & Company, 1980, 204–213, 0-393-30023-4, BOOK, Maynard Smith, John, John Maynard Smith, Evolutionary Genetics, Oxford University Press, 1989, 273–274, 0-19-854215-1, 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.JOURNAL, 10.1093/bjps/20.2.97, Ruse, Michael, Michael Ruse, Definitions of Species in Biology, The British Journal for the Philosophy of Science, 20, 2, 97–119, 1969, 686173, JOURNAL, Lewin, Ralph A., Ralph A. Lewin, Three Species Concepts, Taxon, 1981, 30, 3, 609–613, 10.2307/1219942, In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, 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.Claridge et al.:404. 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.JOURNAL, Ghiselin, Michael T., Michael Ghiselin, A Radical Solution to the Species Problem, Systematic Biology, 23, 4, 536, 1974, 10.1093/sysbio/23.4.536, {{anchor|Recognition species}}{{anchor|Cohesion species}}

Recognition and cohesion species

A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.Claridge et al.:408–409.BOOK, Paterson, H. E. H., 1985, Monograph No. 4: The recognition concept of species, Species and Speciation, Vrba, E. S., Transvaal Museum, Pretoria, 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.BOOK, Mallet, James, Species, Concepts of,weblink Blackwell, Calow, P., Encyclopaedia of Ecology and Environmental Management, 709–711, 978-0-632-05546-3, no,weblink" title="web.archive.org/web/20161019152621weblink">weblink 19 October 2016, dmy-all, A further development of the recognition concept is provided by the biosemiotic concept of species.JOURNAL, Kalevi Kull, Kull, Kalevi, 2016, The biosemiotic concept of the species., Biosemiotics, 9, 61–71,weblink 10.1007/s12304-016-9259-2, no,weblink" title="web.archive.org/web/20180207144820weblink">weblink 7 February 2018, dmy-all, {{anchor|Genetic similarity}}{{anchor|Barcode species}}

Genetic similarity and barcode species

File:Cytochrome C Oxidase 1OCC in Membrane 2.png|thumb|A region of the gene for the cytochrome c oxidase enzyme is used to distinguish species in the Barcode of Life Data SystemsBarcode of Life Data SystemsIn 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 kinds 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 or not.JOURNAL, Stackebrandt, E., Goebel, B. M., Taxonomic note: a place for DNA-DNA reassociation and 16S rRNA sequence analysis in the present species definition in bacteriology, Int. J. Syst. Bacteriol., 1994, 44, 4, 846–849, 10.1099/00207713-44-4-846, This concept was narrowed in 2006 to a similarity of 98.7%.JOURNAL, Stackebrandt, E., Ebers, J., Taxonomic parameters revisited: tarnished gold standards, Microbiology Today, 2006, 33, 4, 152–155,weblink no,weblink 25 March 2018, dmy-all, DNA-DNA hybridisation is outdated, and results have sometimes led to misleading conclusions about species, as with the pomarine and great skua.BOOK, Newton, Ian, Speciation and Biogeography of Birds, Academic Press, 2003, 69, 978-0-08-092499-1,weblink no,weblink 7 February 2018, dmy-all, JOURNAL, Andersson, Malte, 1999, Hybridization and skua phylogeny, Proceedings of the Royal Society B, 266, 1428, 1579–1585, 10.1098/rspb.1999.0818, 1690163,weblink 9780080924991, no,weblink 7 February 2018, dmy-all, Modern approaches compare sequence similarity using computational methods.JOURNAL, Keswani, J., Whitman, W B, 2001, Relationship of 16S rRNA sequence similarity to DNA hybridization in prokaryotes, International Journal of Systematic and Evolutionary Microbiology, 51, 2, 667, 10.1099/00207713-51-2-667, DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use.WEB, What Is DNA Barcoding?,weblink Barcode of Life, 11 October 2017, no,weblink" title="web.archive.org/web/20170701081215weblink">weblink 1 July 2017, dmy-all, The so-called barcode is a region of mitochondrial DNA within the gene for cytochrome c oxidase. A database, Barcode of Life Data Systems (BOLD) contains DNA barcode sequences from over 190,000 species.JOURNAL, Ratnasingham, Sujeevan, Hebert, Paul D. N., BOLD: The Barcode of Life Data System weblink Molecular Ecology Notes, 2007, 7, 3, 355–364, 10.1111/j.1471-8286.2007.01678.x, 18784790, 1890991, JOURNAL, Stoeckle, Mark, DNA Barcoding Ready for Breakout, GeneWatch, November–December 2013, 26, 5,weblink However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider a misnomer, need to be reconciled, as they delimit species differently.JOURNAL, DeSalle, R., Egan, M. G., Siddall, M., The unholy trinity: taxonomy, species delimitation and DNA barcoding, Philosophical Transactions of the Royal Society B: Biological Sciences, 360, 1462, 2005, 1905–1916, 10.1098/rstb.2005.1722, 16214748, 1609226, Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species.JOURNAL, 10.1098/rspb.2007.0062, 17472911, 2493573, DNA barcoding cannot reliably identify species of the blowfly genus Protocalliphora (Diptera: Calliphoridae), Proceedings of the Royal Society B: Biological Sciences, 274, 1619, 1731, 2007, Whitworth, T. L., Dawson, R. D., Magalon, H., Baudry, E., {{anchor|Phylogenetic species}}{{anchor|Cladistic species}}

Phylogenetic, cladistic, or evolutionary species

(File:Cladistic species.svg|thumb|upright|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.Wheeler, Quentin D.; Platnick, Norman I. 2000. The phylogenetic species concept (sensu Wheeler & Platnick). In: Wheeler, Q. D.; Meier, R., editors. Species concepts and phylogenetic theory: a debate. New York: Columbia University Press. pp. 55–69.JOURNAL, Giraud, T., Refrégier, G., Le Gac, M., de Vienne, D.M., Hood, M.E., 2008, Speciation in Fungi, Fungal Genetics and Biology, 45, 6, 791–802, 10.1016/j.fgb.2008.02.001, BOOK, Bernardo, J., 2011, A critical appraisal of the meaning and diagnosability of cryptic evolutionary diversity, and its implications for conservation in the face of climate change, 380–438, Climate Change, Ecology and Systematics. Systematics Association Special Series, T., Hodkinson, M., Jones, S., Waldren, J., Parnell, Cambridge University Press, 978-0-521-76609-8, . 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 mitochondrial DNA of various species.JOURNAL, Nixon, K. C., Wheeler, Q. D., 1990, An amplification of the phylogenetic species concept, Cladistics, 6, 211–223, 10.1111/j.1096-0031.1990.tb00541.x, JOURNAL, Taylor, J.W., Jacobson, D. J., Kroken, S., Kasuga, T., Geiser, D.M., Hibbett, D.S., Fisher, M. C., 2000, Phylogenetic species recognition and species concepts in fungi, Fungal Genetics and Biology, 31, 21–32, 10.1006/fgbi.2000.1228, 11118132, For example, in a study done on fungi, studying the nucleotide characters using cladistic species produced the most accurate results in recognising the numerous fungi species of all the concepts studied.JOURNAL, 10.1098/rstb.2006.1923, Eukaryotic microbes, species recognition and the geographic limits of species: Examples from the kingdom Fungi, Philosophical Transactions of the Royal Society B: Biological Sciences, 361, 1475, 1947, 2006, Taylor, J. W, Turner, E, Townsend, J. P, Dettman, J. R, Jacobson, D, Versions of the Phylogenetic Species Concept may emphasize monophyly or diagnosability.{{sfn|Zachos|2016|pp=91–92}}Unlike the Biological Species Concept, a cladistic species does not rely on reproductive isolation, so it is independent of processes that are integral in other concepts. It works for asexual lineages, and can detect recent divergences, which the Morphological Species Concept cannot. However, it does not work in every situation, and may require more than one polymorphic locus to give an accurate result. The concept may lead to splitting of existing species, for example of Bovidae, into many new ones.Groves, C.; Grubb, P. 2011. Ungulate taxonomy. Baltimore, MD: The Johns Hopkins University Press.JOURNAL, Heller, R., Frandsen, P., Lorenzen, E. D., Siegismund, H. R., 2013, Are there really twice as many bovid species as we thought?,weblink Systematic Biology, 62, 3, 490–493, 10.1093/sysbio/syt004, JOURNAL, Cotterill, F., Taylor, P., Gippoliti, S., 2014, Why one century of phenetics is enough: Response to 'are there really twice as many bovid species as we thought?', Systematic Biology, 63, 5, 819–832, 10.1093/sysbio/syu003, etal, 24415680, 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".JOURNAL, Laporte, L. O. F., Simpson on species, 10.1007/BF01058629, Journal of the History of Biology, 27, 1, 141–159, 1994, 11639257, This differs from the biological species concept in embodying persistence over time. Wiley and Mayden state that they see the evolutionary species concept as "identical" to Willi Hennig's species-as-lineages concept, and assert 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 suggest that the concept works for both asexual and sexually-reproducing species.Wheeler, pp. 70–92, 146–160, 198–208

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.BOOK, Ridley, Mark, Evolution, Blackwell Science, The Idea of Species, 2nd, 719, 0-86542-495-0,

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.JOURNAL, Baker, Robert J., Bradley, Robert D., Speciation in Mammals and the Genetic Species Concept, Journal of Mammalogy, 2006, 87, 4, 643–662, 10.1644/06-MAMM-F-038R2.1,weblink 19890476, 2771874, In the 21st century, a genetic species can be established by comparing DNA sequences, but other methods were available earlier, such as comparing karyotypes (sets of chromosomes) and allozymes (enzyme variants).JOURNAL, Baker, Robert J., Bradley, Robert D., Speciation in Mammals and the Genetic Species Concept, Journal of Mammalogy, 87, 4, 2006, 643–662, 10.1644/06-MAMM-F-038R2.1,

Evolutionarily significant unit

An evolutionarily significant unit (ESU) or "wildlife species"WEB, Government of Canada. Committee on the Status of Endangered Wildlife in Canada,weblink COSEWIC's Assessment Process and Criteria, Cosepac.gc.ca, 7 April 2015, yes,weblink" title="web.archive.org/web/20150412232417weblink">weblink 12 April 2015, dmy-all, is a population of organisms considered distinct for purposes of conservation.WEB, DeWeerdt, Sarah, What Really is an Evolutionarily Significant Unit?,weblink University of Washington, 1 December 2016, 29 July 2002, no,weblink" title="web.archive.org/web/20170205155325weblink">weblink 5 February 2017, dmy-all,

Chronospecies

File:Chronospecies.svg|thumb|upright|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.]]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.WEB, Chronospecies,weblink Oxford Reference, 12 January 2018, no,weblink" title="web.archive.org/web/20141001151656weblink">weblink 1 October 2014, dmy-all, WEB, Carr, Steven M., Evolutionary species and chronospecies,weblink Memorial University Newfoundland and Labrador, 12 January 2018, 2005, no,weblink" title="web.archive.org/web/20160310075417weblink">weblink 10 March 2016, dmy-all, JOURNAL, Dzik, J., 1985, Typologic versus population concepts of chronospecies: implications for ammonite biostratigraphy, Acta Palaeontologica Polonica, 30, 1–2, 71–92,weblink no,weblink" title="web.archive.org/web/20170313080626weblink">weblink 13 March 2017, dmy-all, BOOK, O'Brien, Michael J., Lyman, R. Lee, Applying Evolutionary Archaeology: A Systematic Approach,weblink 2007, Springer, 978-0-306-47468-2, 146–149, no,weblink 7 February 2018, dmy-all,

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.JOURNAL, Van Regenmortel, Marc H.V, Logical puzzles and scientific controversies: The nature of species, viruses and living organisms, Systematic and Applied Microbiology, 33, 1, 1–6, 2010, 10.1016/j.syapm.2009.11.001, 20005655, A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly 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.JOURNAL, van Nimwegen, Erik, Crutchfield, James P., Huynen, Martijn, Neutral evolution of mutational robustness, August 1999, PNAS, 96, 17, 9716–9720,weblink 10.1073/pnas.96.17.9716, JOURNAL, Wilke, Claus O., Wang, Jia Lan, Ofria, Charles, Lenski, Richard E., Adami, Christoph, Evolution of digital organisms at high mutation rates leads to survival of the flattest, Nature, 412, 6844, 2001, 331–333, 10.1038/35085569, JOURNAL, Elena, S. F., Agudelo-Romero, P., Carrasco, P., Codoñer, F. M., Martín, S., Torres-Barceló, C., Sanjuán, R., Experimental evolution of plant RNA viruses, Heredity, 100, 5, 2008, 478–483, 10.1038/sj.hdy.6801088,

Change

Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct.{{sfn|Zachos|2016|pp=77–96}}

Speciation

The evolutionary process by which biological populations evolve to become distinct or reproductively isolated as species is called speciation.JOURNAL, Cook, Orator F., Orator F. Cook, 30 March 1906, Factors of species-formation, Science (journal), Science, Washington, D. C., 23, 587, 506–507, 10.1126/science.23.587.506, 17789700, JOURNAL, Cook, Orator F., Orator F. Cook, November 1908, Evolution Without Isolation, The American Naturalist, 42, 503, 727–731, 10.1086/279001, Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species.JOURNAL, Via, Sara, 16 June 2009, Natural selection in action during speciation,weblink Proceedings of the National Academy of Sciences of the United States of America, Proc. Natl. Acad. Sci. U.S.A., 106, Suppl 1, 9939–9946, 10.1073/pnas.0901397106, 2702801, 19528641, no,weblink" title="web.archive.org/web/20150426115246weblink">weblink 26 April 2015, dmy-all, Speciation depends on a measure of reproductive isolation, a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model.JOURNAL, Barton, N. H., What role does natural selection play in speciation?, Philosophical Transactions of the Royal Society B, June 2010, 365, 1547, 10.1098/rstb.2010.0001, 1825–1840, A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form, without increasing the number of resultant species.JOURNAL, Mayr, Ernst, Ernst Mayr, Speciation and Macroevolution, Evolution, 36, 6, 1982, 1119–1132, 10.1111/j.1558-5646.1982.tb05483.x,

Exchange of genes between species

File:Horizontal-gene-transfer.jpg|thumb|Horizontal gene transfers between widely separated species complicate the phylogeny of bacteriabacteriaHorizontal 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.JOURNAL, Pennisi, Elizabeth, 335, 334–335, Science, 2004, Researchers Trade Insights about Gene Swapping,weblink yes,weblink" title="web.archive.org/web/20060218071130weblink">weblink 18 February 2006, dmy-all, JOURNAL, 10.1016/j.tig.2004.02.004, 15041172,weblink Cladogenesis, coalescence and the evolution of the three domains of life, Trends in Genetics, 20, 4, 182, 2004, Zhaxybayeva, Olga, Peter Gogarten, J., no,weblink" title="web.archive.org/web/20090326181517weblink">weblink 26 March 2009, dmy-all, JOURNAL, 10.1111/bij.12872, Speciation through the looking-glass, Biological Journal of the Linnean Society, 120, 2, 480–488, 2017, Vaux, Felix, Trewick, Steven A., Morgan-Richards, Mary, 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.JOURNAL, Venton, Danielle, Highlight: Applying the Biological Species Concept across All of Life, Genome Biology and Evolution, 9, 3, 2017, 502–503, 10.1093/gbe/evx045, 28391326, 5381533, JOURNAL, Bobay, Louis-Marie, Ochman, Howard, Biological Species Are Universal across Life's Domains, Genome Biology and Evolution, 9, 3, 2017, 491–501, 10.1093/gbe/evx026,

Extinction

{{further|Extinction|Extinction event}}A species is extinct when the last individual of that species dies, but it may be functionally extinct well before that moment. It is estimated that over 99 percent of all species that ever lived on Earth, some five billion species, are now extinct. Some of these were in mass extinctions such as those at the ends of the Permian, Triassic and Cretaceous periods. Mass extinctions had a variety of causes including volcanic activity, climate change, and changes in oceanic and atmospheric chemistry, and they in turn had major effects on Earth's ecology, atmosphere, land surface, and waters.BOOK, Kunin, W.E., Gaston, Kevin, The Biology of Rarity: Causes and consequences of rare—common differences,weblink 1996, 978-0-412-63380-5, no,weblink 5 September 2015, dmy-all, BOOK, Stearns, Beverly Peterson, Stearns, S. C., Stearns, Stephen C., Watching, from the Edge of Extinction,weblink 2000, Yale University Press, 978-0-300-08469-6, preface x, 30 May 2017, no,weblink 17 July 2017, dmy-all, Another form of extinction is through the assimilation of one species by another through hybridization. The resulting single species has been termed as a "compilospecies".{{sfn|Zachos|2016|p=82}}

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. Paul Michael-Agapow and colleagues found that surveys using a phylogenetic species concept reported 48% more species and accordingly smaller populations and ranges than those using nonphylogenetic concepts; they note that this "taxonomic inflation"JOURNAL, 10.1111/jzs.12088, Taxonomic inflation, the Phylogenetic Species Concept and lineages in the Tree of Life – a cautionary comment on species splitting, Journal of Zoological Systematics and Evolutionary Research, 53, 2, 180, 2015, Zachos, Frank E., could cause a false appearance of change to the number of endangered species and consequent political and practical difficulties.JOURNAL, Paul‐Michael, Agapow, Olaf R. P., Bininda‐Emonds, Keith A, Crandall, John L., Gittleman, Georgina M., Mace, Jonathon C., Marshall, Andy, Purvis, The Quarterly Review of Biology, 79, 2, 2004, 161–179, The Impact of Species Concept on Biodiversity Studies,weblink 10.1086/383542, 10.1086/383542, no,weblink" title="web.archive.org/web/20180111110303weblink">weblink 11 January 2018, dmy-all, The evolutionary biologist Jody Hey observes that there is an inherent conflict between the desire to understand the processes of speciation and the need to identify and to categorise.JOURNAL, Hey, Jody, July 2001, The mind of the species problem, Trends in Ecology and Evolution, 16, 7, 326–329, 10.1016/S0169-5347(01)02145-0, 11403864, 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.BOOK,weblink The Endangered Species Act at Thirty, Volume 2: Conserving Biodiversity in Human-Dominated Landscapes, Haig, Susan M., Allendorf, F. W., Island Press, 2006, Scott, J. Michael, Washington, 150–163, Hybrids and Policy, Goble, D. D., Frank W. Davis, no,weblink 7 February 2018, dmy-all, The botanist Brent D. Mishler states that the species problem is created by the many ways that people want to use the species category, but argues that the solution is to abandon the traditional ranks and just use monophyletic groups of different inclusivess. A species is then "simply the least inclusive taxon" of whatever type. He argues that ecology, evolution, and conservation work are all better served by this approach. Since in his view species are not comparable, counting them is not a valid measure of biodiversity, and he calls for new ways to measure it, noting that other authors such as R. I. Vane-Wright and colleaguesJOURNAL, Vane-Wright, R. I., Humphries, C. J., Williams, P. H., 1991, What to protect? – systematics and the agony of choice, Biological Conservation, 55, 3, 235–254, 10.1016/0006-3207(91)90030-D, have proposed quantitative measures for phylogenetic biodiversity that make use of "the number of branch points, and possibly branch lengths, separating the tips on the tree".BOOK, Mishler, Brent D., Wilson, R., Getting Rid of Species?, Species: New Interdisciplinary Essays, 1999, MIT Press, 307–315,weblink 978-0262731232, no,weblink 10 January 2018, dmy-all,

See also

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Notes

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Cited sources

  • BOOK, Claridge, M. F., Dawah, H. A., Wilson, M. R., Chapman & Hall, Species. The units of biodiversity, 1997, harv,
  • BOOK, Wheeler, Quentin, Meier, Rudolf, Species concepts and phylogenetic theory : a debate, Columbia University Press, New York, 2000, 978-0-231-10143-1,
  • BOOK, Zachos, Frank E., Species concepts in biology. Historical Development, Theoretical Foundations and Practical Relevance, Springer International Publishing, 2016, harv,

External links

{{Wikispecies}}{{Commons category}}{{Wiktionary}} {{Speciation|state=open}}{{Taxonomic ranks|state=open}}{{Use dmy dates|date=January 2018}}

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