Foraminifera are members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses. Tests of chitin are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment, while a smaller variety float in the water column at various depths. Fewer are known from freshwater or brackish conditions, some few soil species have been identified through molecular analysis of small subunit ribosomal DNA. Foraminifera produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure; these shells are made of calcium carbonate or agglutinated sediment particles. Over 50,000 species are recognized, both fossil, they are less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm. In modern Scientific English, the term foraminifera is both singular and plural, is used to describe one or more specimens or taxa: its usage as singular or plural must be determined from context.
Foraminifera is used informally to describe the group, in these cases is lowercase. The taxonomic position of the Foraminifera has varied since their recognition as protozoa by Schultze in 1854, there referred to as an order, Foraminiferida. Loeblich and Tappan reranked Foraminifera as a class as it is now regarded; the Foraminifera have been included in the Protozoa, or in the similar Protoctista or Protist kingdom. Compelling evidence, based on molecular phylogenetics, exists for their belonging to a major group within the Protozoa known as the Rhizaria. Prior to the recognition of evolutionary relationships among the members of the Rhizaria, the Foraminifera were grouped with other amoeboids as phylum Rhizopodea in the class Granuloreticulosa; the Rhizaria are problematic, as they are called a "supergroup", rather than using an established taxonomic rank such as phylum. Cavalier-Smith defines the Rhizaria as an infra-kingdom within the kingdom Protozoa; some taxonomies put the Foraminifera in a phylum of their own, putting them on par with the amoeboid Sarcodina in which they had been placed.
Although as yet unsupported by morphological correlates, molecular data suggest the Foraminifera are related to the Cercozoa and Radiolaria, both of which include amoeboids with complex shells. However, the exact relationships of the forams to the other groups and to one another are still not clear. Foraminifera are related to testate amoebae; the most recent taxonomy by Mikhalevich 2013. Foraminifera d'Orbigny 1826 Order Reticulomyxida Class Schizocladea Cedhagen & Mattson 1992 Order Schizocladida Class Xenophyophorea Schultze 1904 Order Stannomida Tendal 1972 Order Psamminida Tendal 1972 Class Astrorhizata Saidova 1981 Subclass Lagynana Mikhalevich 1980 Order Ammoscalariida Mikhalevich 1980 Order Lagynida Mikhalevich 1980 Order Allogromiida Loeblich & Tappan 1961 Subclass Astrorhizana Saidova 1981 Order Astrorhizida Lankester 1885 Order Dendrophryida Mikhalevich 1995 Order Hippocrepinida Saidova 1981 Order †Parathuramminida Mikhalevich 1980 Order Psammosphaerida Haeckel 1894 Class Rotaliata Mikhalevich 1980 Subclass Globigerinana Mikhalevich 1980 Order Cassigerinellida Mikhalevich 2013 Order Globigerinida Carpenter, Parker & Jones 1862 Order Hantkeninida Mikhalevich 1980 Order Heterohelicida Fursenko 1958 Order Globorotaliida Mikhalevich 1980 Subclass Textulariana Mikhalevich 1980 Order Nautiloculinida Mikhalevich 2003 Order Spiroplectamminida Mikhalevich 1992 Order Textulariida Delage & Hérouard 1896 Order Trochamminida Saidova 1981 (Carterinida Loeblich & Tappan 1955] Order Verneuilinida Mikhalevich & Kaminski 2003 Subclass Rotaliana Mikhalevich 1980 Superorder Robertinoida Mikhalevich 1980 Order Robertinida Mikhalevich 1980 Superorder Nonionoida Saidova 1981 Order Elphidiida Saidova 1981 Order Nummulitida Carpenter, Parker & Jones 1862 Order †Orbitoidida Copeland 1956 Order Nonionida Saidova 1981 Superorder Buliminoida Saidova 1981 Order Cassidulinida d’Orbigny 1839 Order Buliminida Saidova 1981 Order Bolivinitida Saidova 1981 Superorder Discorboida Ehrenberg 1838 Order Chilostomellida Haeckel 1894 Order Discorbida Ehrenberg 1838 Order Glabratellida Mikhalevich 1994 Order Planorbulinida Mikhalevich 1992 Order Rotaliida Lankester 1885 Order Rosalinida Delage & Hérouard 1896 Class Nodosariata Mikhalevich 1992 Subclass Hormosinana Mikhalevich 1992 Order Ammomarginulinida Mikhalevich 2002 Order Nouriida Mikhalevich 1980 Order †Pseudopalmulida Mikhalevich 1992 Order Saccamminida Lankester 1885 Order Hormosinida Mikhalevich 1980 Subclass Nodosariana Mikhalevich 1992 Order †Biseriamminida Mikhalevich 1981 Order Delosinida Revets 1989 Order Lagenida Delage & Hérouard 1896 Order †Palaeotextulariida Hohenegger & Piller 1975 Order Polymorphinida Mikhalevich 1980 Order Vaginulinida Mikhalevich 1993 Order Nodosariida Calkins 1926 Class Spirillinata Mikhalevich 1992 Subclass Ammodiscana Mikhalevich 1980 Order †Plagioraphida Mikhalevich 2003 Order Ammodiscida Mikhalevich 1980 Order Ammovertellinida Mikhalevich 1999 Order Ataxophragmiida Fursenko 1958 Subclass Spirillinana Mikhalevich 1992 Superorder †Archaediscoida Pojarkov & Skvortsov 1979 Order †Archaediscida Pojarkov & Skvortsov 1979 Order †Lasiodiscida
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to Eukarya, their name comes from the Greek εὖ and κάρυον. Eukaryotic cells contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells; these act as sex cells. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, makes up one of the domains of life in the three-domain system. The two other domains and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved 1.6–2.1 billion years ago, during the Proterozoic eon. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton; the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1937 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes; however he mentioned this in only one paragraph, the idea was ignored until Chatton's statement was rediscovered by Stanier and van Niel.
In 1905 and 1910, the Russian biologist Konstantin Mereschkowski argued that plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, On the origin of mitosing cells. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA; this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles and chloroplasts. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's nucleus came from the ability of Gram-positive bacteria to form endospores. In 1987 and papers, Thomas Cavalier-Smith proposed instead that the membranes of the nucleus and endoplasmic reticulum first formed by infolding a prokaryote's plasma membrane.
In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus reviving Mereschkowski's theory. Eukaryotic cells are much larger than those of prokaryotes having a volume of around 10,000 times greater than the prokaryotic cell, they have a variety of internal membrane-bound structures, called organelles, a cytoskeleton composed of microtubules and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and pinches off to form a vesicle, it is probable that most other membrane-bound organelles are derived from such vesicles.
Alternatively some products produced by the cell can leave in a vesicle through exocytosis. The nucleus is surrounded with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, involved in protein transport and maturation, it includes the rough endoplasmic reticulum where ribosomes are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, the Golgi apparatus. Vesicles may be specialized for various purposes. For instance, lysosomes contain digestive enzymes that break down most biomolecules in the cytoplasm. Peroxisomes are used to break down peroxide, otherwise toxic. Many protozoans have contractile vacuoles, which collect and expel excess water, extrusomes, which expel material used to deflect predators or capture prey.
In higher plants, most of a cell's volume is taken up by a central vacuole, whi
An amoeba called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape by extending and retracting pseudopods. Amoebas do not form a single taxonomic group. Amoeboid cells occur not only among the protozoa, but in fungi and animals. Microbiologists use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement. In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Amoeboid organisms are no longer classified together in one group; the best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, the multicellular "social amoeba" or slime mould Dictyostelium discoideum.
Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba have bulbous pseudopods, rounded at the ends and tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have thread-like pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like structures; some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia supported from within by bundles of microtubules. Free-living amoebae may be "testate", or "naked"; the shells of testate amoebae may be composed of various substances, including calcium, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms. To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.
This organelle is necessary because freshwater has a lower concentration of solutes than the amoeba's own internal fluids. Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and burst. Marine amoebae do not possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water; the food sources of amoebae vary. Some amoebae are live by consuming bacteria and other protists; some eat dead organic material. Amoebae ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, there is no fixed place on the cell at which phagocytosis occurs; some amoebae feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
The size of amoeboid cells and species is variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter, within the size range of many bacteria. At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter. Most of the free-living freshwater amoebae found in pond water and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye; some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, engulf them by phagocytosis. Amoeboid stages occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds classified in the class Myxogastria, the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage.
The amoeboid cells of the former combine to form a giant multinucleate organism, while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism. Other organisms may present amoeboid cells during certain life-cycle stages, e.g. the gametes of some green algae and pennate diatoms, the spores of some Mesomycetozoea, the sporoplasm stage of Myxozoa and of Ascetosporea. The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus". Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus; the term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid. In 1822, the genus Amiba was erected by the Frenc
Wikispecies is a wiki-based online project supported by the Wikimedia Foundation. Its aim is to create a comprehensive free content catalogue of all species. Jimmy Wales stated that editors are not required to fax in their degrees, but that submissions will have to pass muster with a technical audience. Wikispecies is available under the GNU Free Documentation License and CC BY-SA 3.0. Started in September 2004, with biologists across the world invited to contribute, the project had grown a framework encompassing the Linnaean taxonomy with links to Wikipedia articles on individual species by April 2005. Benedikt Mandl co-ordinated the efforts of several people who are interested in getting involved with the project and contacted potential supporters in early summer 2004. Databases were evaluated and the administrators contacted, some of them have agreed on providing their data for Wikispecies. Mandl defined two major tasks: Figure out how the contents of the data base would need to be presented—by asking experts, potential non-professional users and comparing that with existing databases Figure out how to do the software, which hardware is required and how to cover the costs—by asking experts, looking for fellow volunteers and potential sponsorsAdvantages and disadvantages were discussed by the wikimedia-I mailing list.
The board of directors of the Wikimedia Foundation voted by 4 to 0 in favor of the establishment of a Wikispecies. The project is hosted at species.wikimedia.org. It was merged to a sister project of Wikimedia Foundation on September 14, 2004. On October 10, 2006, the project exceeded 75,000 articles. On May 20, 2007, the project exceeded 100,000 articles with a total of 5,495 registered users. On September 8, 2008, the project exceeded 150,000 articles with a total of 9,224 registered users. On October 23, 2011, the project reached 300,000 articles. On June 16, 2014, the project reached 400,000 articles. On January 7, 2017, the project reached 500,000 articles. On October 30, 2018, the project reached 600,000 articles, a total of 1.12 million pages. Wikispecies comprises taxon pages, additionally pages about synonyms, taxon authorities, taxonomical publications, institutions or repositories holding type specimen. Wikispecies asks users to use images from Wikimedia Commons. Wikispecies does not allow the use of content.
All Species Foundation Catalogue of Life Encyclopedia of Life Tree of Life Web Project List of online encyclopedias The Plant List Wikispecies, The free species directory that anyone can edit Species Community Portal The Wikispecies Charter, written by Wales
The Southern Ocean known as the Antarctic Ocean or the Austral Ocean, comprises the southernmost waters of the World Ocean taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the fourth largest of the five principal oceanic divisions: smaller than the Pacific and Indian Oceans but larger than the Arctic Ocean; this ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters. By way of his voyages in the 1770s, Captain James Cook proved that waters encompassed the southern latitudes of the globe. Since geographers have disagreed on the Southern Ocean's northern boundary or existence, considering the waters as various parts of the Pacific and Indian Oceans, instead. However, according to Commodore John Leech of the International Hydrographic Organization, recent oceanographic research has discovered the importance of Southern Circulation, the term Southern Ocean has been used to define the body of water which lies south of the northern limit of that circulation.
This remains the current official policy of the IHO, since a 2000 revision of its definitions including the Southern Ocean as the waters south of the 60th parallel has not yet been adopted. Others regard the seasonally-fluctuating Antarctic Convergence as the natural boundary; the maximum depth of the Southern Ocean, using the definition that it lies south of 60th parallel, was surveyed by the Five Deeps Expedition in early February 2019. The expedition's multibeam sonar team identified the deepest point at 60° 28' 46"S, 025° 32' 32"W, with a depth of 7,434 meters; the expedition leader and chief submersible pilot Victor Vescovo, has proposed naming this deepest point in the Southern Ocean the "Factorian Deep," based on the name of the manned submersible DSV Limiting Factor, in which he visited the bottom for the first time on February 3, 2019. Borders and names for oceans and seas were internationally agreed when the International Hydrographic Bureau, the precursor to the IHO, convened the First International Conference on 24 July 1919.
The IHO published these in its Limits of Oceans and Seas, the first edition being 1928. Since the first edition, the limits of the Southern Ocean have moved progressively southwards; the IHO included the ocean and its definition as the waters south of 60°S in its year 2000 revisions, but this has not been formally adopted, due to continuing impasses over other areas of the text, such as the naming dispute over the Sea of Japan. The 2000 IHO definition, was circulated in a draft edition in 2002 and is used by some within the IHO and by some other organizations such as the US Central Intelligence Agency and Merriam-Webster. Australian authorities regard the Southern Ocean as lying south of Australia; the National Geographic Society does not recognize the ocean, depicting it in a typeface different from the other world oceans. Map publishers using the term Southern Ocean on their maps include Hema GeoNova. "Southern Ocean" is an obsolete name for the Pacific Ocean or South Pacific, coined by Vasco Núñez de Balboa, the first European to discover it, who approached it from the north.
The "South Seas" is a less archaic synonym. A 1745 British Act of Parliament established a prize for discovering a Northwest Passage to "the Western and Southern Ocean of America". Authors using "Southern Ocean" to name the waters encircling the unknown southern polar regions used varying limits. James Cook's account of his second voyage implies. Peacock's 1795 Geographical Dictionary said it lay "to the southward of America and Africa"; the Family Magazine in 1835 divided the "Great Southern Ocean" into the "Southern Ocean" and the "Antarctick Ocean" along the Antarctic Circle, with the northern limit of the Southern Ocean being lines joining Cape Horn, the Cape of Good Hope, Van Diemen's Land and the south of New Zealand. The United Kingdom's South Australia Act 1834 described the waters forming the southern limit of the new colony of South Australia as "the Southern Ocean"; the Colony of Victoria's Legislative Council Act of 1881 delimited part of the division of Bairnsdale as "along the New South Wales boundary to the Southern ocean".
In the 1928 first edition of Limits of Oceans and Seas, the Southern Ocean was delineated by land-based limits: Antarctica to the south, South America, Africa and Broughton Island, New Zealand to the north. The detailed land-limits used were from Cape Horn in Chile eastwards to Cape Agulhas in Africa further eastwards to the southern coast of mainland Australia to Cape Leeuwin, Western Australia. From Cape Leeuwin, the limit followed eastwards along the coast of mainland Australia to Cape Otway, Victoria southwards across Bass Strait to Cape Wickham, King Island, along the west coast of King Island the remainder of the way south across Bass Strait to Cape Grim, Tasmania; the limit followed the west coast of Tasmania southwards to the South East Cape and went eastwards to Broughton Island, New Zealand, before returning to Cape Horn. The northern limits of the Southern Ocean were moved southwards in the IHO's 1937 second edition of the Limits of Oceans and Seas. From this edition, much of the ocean's northern limit ceased to abut land masses.
In the second edition, the Southern Ocean extended from Antarctica northwards to latitude 40°S between Cape Agulhas
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. A multicellular form has been described; this supergroup was proposed by Cavalier-Smith in 2002. Being described from rDNA sequences, they vary in form, having no clear morphological distinctive characters, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, these make up the vast majority of protozoan fossils. Nearly all have mitochondria with tubular cristae; the three main groups of Rhizaria are: Cercozoa – various amoebae and flagellates with filose pseudopods and common in soil Foraminifera – amoeboids with reticulose pseudopods, common as marine benthos Radiolaria – amoeboids with axopods, common as marine planktonA few other groups may be included in the Cercozoa, but on some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals and the peculiar amoeba Gromia.
The different groups of Rhizaria are considered close relatives based on genetic similarities, have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by Cavalier-Smith in 2002, who included the centrohelids and Apusozoa. Another order that appears to belong to this taxon is the Mikrocytida; these are parasites of oysters. Rhizaria are part of the Diaphoretickes clade along with Archaeplastida, Cryptista and Halvaria. Many rhizarians were considered animals because of their motility and heterotrophy. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists placed rhizarians in the kingdom Protista; when scientists began examining the evolutionary relationships among eukaryotes using molecular data, it became clear that the kingdom Protista was paraphyletic. Rhizaria appear to share a common ancestor with Stramenopiles and Alveolates forming part of the SAR super assemblage. Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group.
Biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests a relevant ecological role during the Ediacaran. Phylogeny based on al.. 2009, Howe et al. 2011, Silar 2016. In 2019, the Cercozoa were recognized as sister of the Retaria. Molecular Phylogeny of Amoeboid Protists - Tree of Rhizaria Tree of Life Eukaryotes
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, to grade into one another. 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. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was 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. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. 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 formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. 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, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens 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 different species. Species named in this manner are called morphospecies. 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, 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. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria 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. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri