Kingdom (biology)
In biology, a kingdom is the highest taxonomic rank canonized by international taxonomic committees, although in common usage the newer rank of domain is recognized as above it, making kingdom the second-highest rank. Below the level of kingdom is Phylum, of which there are more than fifty taxa recognized. Some proposals for taxonomic ranks add one above domain, in order to accommodate non-cellular life viruses.
The division of living organisms into kingdoms has a long history, with the "father of modern taxonomy," Carl Linnaeus (1707-1778), formally dividing organisms into two kingdoms: an animal kingdoms (Regnum Animale) and a plant kingdom (Regnum Vegetabile, vegetable kingdom). Two millennium earlier, Aristotle (384–322 B.C.E.) and his pupil Theophrastus (c. 371–c. 287 B.C.E.) had already seen a natural division into animals and plants.
More recently, five- and six-kingdom taxonomies have been popular. The six kingdoms are Animalia, Plantae, Fungi, Protista, Archaea/Archaebacteria, and Bacteria or Eubacteria, whereas the five kingdoms include Animalia, Plantae, Fungi, Protista and Monera. For those who recognize domains as the highest taxonomic unit, the three-domains proposals make the basic division of living (cellular) organisms into the domains of Bacteria, Archaea, and Eukarya (Eucarya), while the two-domain system recognizes Archaea and Bacteria, with Eukarya included in Archaea. When the three-domain system is used, the Eukarya commonly are divided into the Kingdoms of Animal, Plant, Fungi, and Protista.
Some recent classifications based on modern cladistics have explicitly abandoned the term kingdom, noting that some traditional kingdoms are not monophyletic, meaning that they do not consist of all the descendants of a common ancestor.
Just as people often find interest in tracing their own genealogy, they seek to understand their relatedness to other taxa. Darwin wrote in On the Origin of Species, "our classifications will come to be, as far as they can be so made, genealogies." Current taxonomies pursue just such an avenue by seeking to understand connectedness of all life via the evolutionary concept of descent with modification.
Overview
When Carl Linnaeus introduced the rank-based system of nomenclature into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks: class, order, genus and species. In 1990, the rank of domain was introduced as a rank above kingdom (Woese et al. 1990), although Van der Gulik et al. (2023) note that the taxonomic rank of domain "was never canonized by international taxonomic committees."
Prefixes can be added so subkingdom (subregnum) and infrakingdom (also known as infraregnum) are the two ranks immediately below kingdom. Superkingdom may be considered as an equivalent of domain or empire or as an independent rank between kingdom and domain or subdomain. In some classification systems the additional rank branch (Latin: ramus) can be inserted between subkingdom and infrakingdom, e.g., Protostomia and Deuterostomia in the classification of Cavalier-Smith (1998).
For two centuries, from the mid-eighteenth century until the mid-twentieth century, organisms were generally considered to belong to one of two kingdoms, Plantae (plants, including bacteria) or Animalia (animals, including protozoa). Linnaeus used this division as the top rank, dividing the physical world into the vegetable, animal, and mineral kingdoms. This system of dividing living organisms into two kingdoms, as proposed by Carolus Linnaeus in the mid-eighteenth century, had obvious difficulties, including the problem of placing fungi, protists, and prokaryotes. There are single-celled organisms that fall between the two categories, such as Euglena, that can photosynthesize food from sunlight and, yet, feed by consuming organic matter.
As advances in microscopy made the classification of microorganisms possible, the number of kingdoms increased, five- and six-kingdom systems being the most common.
In 1969, American ecologist Robert H. Whittaker proposed a system with five kingdoms: Monera (prokaryotes—bacteria and blue-green algae), Protista (unicellular, multicellular, and colonial protists), Fungi, Plantae, and Animalia. This system was widely used for three decades, but is largely abandoned today (van der Gulik 2023).
Domains are a relatively new grouping. First proposed in 1977, and elaborated on by Woese et al. in 1990, Carl Woese's three-domain system was not generally accepted until later. Also called a "Superregnum" or "Superkingdom," one of the reasons this top-level grouping was advanced was because research revealed the unique nature of anaerobic bacteria (called Archaeobacteria, or simply Archaea). These "living fossils" are genetically and metabolically very different from oxygen-breathing organisms. One main characteristic of the three-domain method is the separation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (a kingdom also sometimes called Monera), with the Eukaryota for all organisms whose cells contain a nucleus (Cracraft and Donaghue 2004). Thus, in the three-domain system, the three groupings are: Archaea; Bacteria; and Eukaryota, emphasizing the separation of prokaryotes into two groups, the Bacteria (originally labeled Eubacteria) and the Archaea (originally labeled Archaebacteria).
In some classifications, authorities keep the kingdom as the higher-level classification, rather than domain, but recognize a sixth kingdom, the Archaebacteria or Archaea.
In summary, today there are several competing top classifications of life. Among these are (a fuller chart is presented below in the section Summary):
- The three-domain system of Carl Woese, with top-level groupings of Archaea, Eubacteria, and Eukaryota domains
- The two-empire system, with top-level groupings of Prokaryota (or Monera) and Eukaryota empires
- The five-kingdom system with top-level groupings of Monera, Protista, Fungi, Plantae, and Animalia
- The six-kingdom system with top-level groupings of Archaebacteria, Monera, Protista, Fungi, Plantae, and Animalia
| Linnaeus 1735 2 kingdoms |
Haeckel 1866 3 kingdoms |
Chatton 1937 2 empires |
Copeland 1956 4 kingdoms |
Whittaker 1969 5 kingdoms |
Woese et al. 1977 6 kingdoms |
Woese et al. 1990 3 domains |
|---|---|---|---|---|---|---|
| (not treated) | Protista | Prokaryota | Monera | Monera | Eubacteria | Bacteria |
| Archaebacteria | Archaea | |||||
| Eukaryota | Protista | Protista | Protista | Eukarya | ||
| Vegetabilia | Plantae | Fungi | Fungi | |||
| Plantae | Plantae | Plantae | ||||
| Animalia | Animalia | Animalia | Animalia | Animalia |
History
Two kingdoms of life
The classification of living things into animals and plants is an ancient one. Aristotle (384–322 B.C.E.) classified animal species in his History of Animals, while his pupil Theophrastus (c. 371–c. 287 B.C.E.) wrote a parallel work, the Historia Plantarum, on plants (Singer 1931).
Carl Linnaeus (1707–1778) laid the foundations for modern biological nomenclature, now regulated by the Nomenclature Codes, in 1735. He distinguished two kingdoms of living things: Regnum Animale ('animal kingdom') and Regnum Vegetabile ('vegetable kingdom', for plants). Linnaeus also included minerals in his classification system, placing them in a third kingdom, Regnum Lapideum.
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Three kingdoms of life
In 1674, Antonie van Leeuwenhoek, often called the "father of microscopy," sent the Royal Society of London a copy of his first observations of microscopic single-celled organisms. Until then, the existence of such microscopic organisms was entirely unknown. Despite this, Linnaeus did not include any microscopic creatures in his original taxonomy.
At first, microscopic organisms were classified within the animal and plant kingdoms. However, by the mid–19th century, it had become clear to many that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded" (Scamardella 1999).
In 1860, John Hogg proposed the Protoctista, a third kingdom of life composed of "all the lower creatures, or the primary organic beings"; he retained Regnum Lapideum as a fourth kingdom of minerals (Scamardella 1999). In 1866, Ernst Haeckel also proposed a third kingdom of life, the Protista, for "neutral organisms" or "the kingdom of primitive forms," which were neither animal nor plant; he did not include the Regnum Lapideum in his scheme (Scamardella 1999). Haeckel revised the content of this kingdom a number of times before settling on a division based on whether organisms were unicellular (Protista) or multicellular (animals and plants) (Scamardella 1999).
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Four kingdoms
The development of microscopy revealed important distinctions between those organisms whose cells do not have a distinct nucleus (prokaryotes) and organisms whose cells do have a distinct nucleus (eukaryotes). In 1937, Édouard Chatton introduced the terms "prokaryote" and "eukaryote" to differentiate these organisms (Sapp 2005).
In 1938, Herbert F. Copeland proposed a four-kingdom classification by creating the novel Kingdom Monera of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified as Bacteria and Archaea. Ernst Haeckel, in his 1904 book The Wonders of Life, had placed the blue-green algae (or Phycochromacea) in Monera; this would gradually gain acceptance, and the blue-green algae would become classified as bacteria in the phylum Cyanobacteria (Scamardella 1999; Sapp 2005).
In the 1960s, Roger Stanier and C. B. van Niel promoted and popularized Édouard Chatton's earlier work, particularly in their paper of 1962, "The Concept of a Bacterium"; this created, for the first time, a rank above kingdom—a superkingdom or empire—with the two-empire system of prokaryotes and eukaryotes (Sapp 2005). The two-empire system would later be expanded to the three-domain system of Archaea, Bacteria, and Eukaryota.
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Five kingdoms
The differences between fungi and other organisms regarded as plants had long been recognised by some; Haeckel had moved the fungi out of Plantae into Protista after his original classification (Scamardella 1999), but was largely ignored in this separation by scientists of his time. Robert Whittaker recognized an additional kingdom for the Fungi (Whittaker 1969). The resulting five-kingdom system, proposed in 1969 by Whittaker, became e a popular standard and with some refinement is still used in many works and forms the basis for new multi-kingdom systems. It is based mainly upon differences in nutrition; his Plantae were mostly multicellular autotrophs, his Animalia multicellular heterotrophs, and his Fungi multicellular saprotrophs.
The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies (Whittaker 1969).The five kingdom system may be combined with the two empire system. In the Whittaker system, Plantae included some algae. In other systems, such as Lynn Margulis's system of five kingdoms, the plants included just the land plants (Embryophyta), and Protoctista has a broader definition (Margulis and Chapman 2009).
Following publication of Whittaker's system, the five-kingdom model began to be commonly used in high school biology textbooks. But despite the development from two kingdoms to five among most scientists, some authors as late as 1975 continued to employ a traditional two-kingdom system of animals and plants, dividing the plant kingdom into subkingdoms Prokaryota (bacteria and cyanobacteria), Mycota (fungi and supposed relatives), and Chlorota (algae and land plants) (Palmer and Fowler 1975).
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Kingdom Monera
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Kingdom Protista
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Kingdom Plantae |
Kingdom Fungi
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Kingdom Animalia
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Six kingdoms
In 1977, Carl Woese and colleagues proposed the fundamental subdivision of the prokaryotes into the Eubacteria (later called the Bacteria) and Archaebacteria (later called the Archaea), based on ribosomal RNA structure (Balch et al. 1977; Woese and Fox 1977); this would later lead to the proposal of three "domains" of life, of Bacteria, Archaea, and Eukaryota (Woese et al. 1990). Combined with the five-kingdom model, this created a six-kingdom model, where the kingdom Monera is replaced by the kingdoms Bacteria and Archaea. This six-kingdom model has been used in US high school biology textbooks, but has received criticism for compromising the current scientific consensus (Case 2008). But the division of prokaryotes into two kingdoms remains in use with the recent seven-kingdoms scheme of Thomas Cavalier-Smith (below), although it primarily differs in that Protista is replaced by Protozoa and Chromista (Ruggiero et al. 2015).
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Eight kingdoms
Thomas Cavalier-Smith supported the consensus at that time, that the difference between Eubacteria and Archaebacteria was so great (particularly considering the genetic distance of ribosomal genes) that the prokaryotes needed to be separated into two different kingdoms. He then divided Eubacteria into two subkingdoms: Negibacteria (Gram-negative bacteria) and Posibacteria (Gram-positive bacteria). Technological advances in electron microscopy allowed the separation of the Chromista from the Plantae kingdom. Indeed, the chloroplast of the chromists is located in the lumen of the endoplasmic reticulum instead of in the cytosol. Moreover, only chromists contain chlorophyll c. Since then, many non-photosynthetic phyla of protists, thought to have secondarily lost their chloroplasts, were integrated into the kingdom Chromista.
Finally, some protists lacking mitochondria were discovered (Cavalier-Smith 1987). As mitochondria were known to be the result of the endosymbiosis of a proteobacterium, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in eukaryogenesis. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdom Archezoa. This superkingdom was opposed to the Metakaryota superkingdom, grouping together the five other eukaryotic kingdoms (Animalia, Protozoa, Fungi, Plantae and Chromista). This was known as the Archezoa hypothesis, which has since been abandoned (Poole and Penny 2007); later schemes did not include the Archezoa–Metakaryota divide (Cavalier-Smith 1998; Ruggiero et al. 2015).
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‡ No longer recognized by taxonomists.
Six kingdoms (1998)
In 1998, Cavalier-Smith published a six-kingdom model, which has been revised in subsequent papers. The version published in 2010 is shown below (Cavalier-Smith 2010). Compared to the version Cavalier-Smith published in 2004 (Cavalier-Smith 2004), the alveolates and the rhizarians have been moved from Kingdom Protozoa to Kingdom Chromista. Cavalier-Smith no longer accepted the importance of the fundamental Eubacteria–Archaebacteria divide put forward by Woese and others and supported by recent research (Dagan et al. 2010). The kingdom Bacteria (sole kingdom of empire Prokaryota) was subdivided into two sub-kingdoms according to their membrane topologies: Unibacteria and Negibacteria. Unibacteria was divided into phyla Archaebacteria and Posibacteria; the bimembranous-unimembranous transition was thought to be far more fundamental than the long branch of genetic distance of Archaebacteria, viewed as having no particular biological significance.
Cavalier-Smith does not accept the requirement for taxa to be monophyletic ("holophyletic" in his terminology) to be valid. He defines Prokaryota, Bacteria, Negibacteria, Unibacteria, and Posibacteria as valid paraphyla ("monophyletic" in the sense he uses this term) taxa, marking important innovations of biological significance (in regard of the concept of biological niche).
In the same way, his paraphyletic kingdom Protozoa includes the ancestors of Animalia, Fungi, Plantae, and Chromista. The advances of phylogenetic studies allowed Cavalier-Smith to realize that all the phyla thought to be archezoans (i.e. primitively amitochondriate eukaryotes) had in fact secondarily lost their mitochondria, typically by transforming them into new organelles: Hydrogenosomes. This means that all living eukaryotes are in fact metakaryotes, according to the significance of the term given by Cavalier-Smith. Some of the members of the defunct kingdom Archezoa, like the phylum Microsporidia, were reclassified into kingdom Fungi. Others were reclassified in kingdom Protozoa, like Metamonada which is now part of infrakingdom Excavata.
Because Cavalier-Smith allows paraphyly, the diagram below is an "organization chart," not an "ancestor chart," and does not represent an evolutionary tree.
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Seven kingdoms
Cavalier-Smith and his collaborators revised their classification in 2015. In this scheme they introduced two superkingdoms of Prokaryota and Eukaryota and seven kingdoms. Prokaryota have two kingdoms: Bacteria and Archaea. (This was based on the consensus in the Taxonomic Outline of Bacteria and Archaea, and the Catalogue of Life). The Eukaryota have five kingdoms: Protozoa, Chromista, Plantae, Fungi, and Animalia. In this classification a protist is any of the eukaryotic unicellular organisms (Ruggiero et al. 2015).
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Summary
| Linnaeus 1735[1] |
Haeckel 1866[2] |
Chatton 1925[3][4] |
Copeland 1938[5][6] |
Whittaker 1969[7] |
Woese et al. 1977[8][9] |
Woese et al. 1990[10] |
Cavalier-Smith 1993[11][12][13] |
Cavalier-Smith 1998[14][15][16] |
Ruggiero et al. 2015[17] |
|---|---|---|---|---|---|---|---|---|---|
| — | — | 2 empires | 2 empires | 2 empires | 2 empires | 3 domains | 3 superkingdoms | 2 empires | 2 superkingdoms |
| 2 kingdoms | 3 kingdoms | — | 4 kingdoms | 5 kingdoms | 6 kingdoms | — | 8 kingdoms | 6 kingdoms | 7 kingdoms |
| — | Protista | Prokaryota | Monera | Monera | Eubacteria | Bacteria | Eubacteria | Bacteria | Bacteria |
| Archaebacteria | Archaea | Archaebacteria | Archaea | ||||||
| Eukaryota | Protista | Protista | Protista | Eucarya | Archezoa | Protozoa | Protozoa | ||
| Protozoa | |||||||||
| Chromista | Chromista | Chromista | |||||||
| Vegetabilia | Plantae | Plantae | Plantae | Plantae | Plantae | Plantae | Plantae | ||
| Fungi | Fungi | Fungi | Fungi | Fungi | |||||
| Animalia | Animalia | Animalia | Animalia | Animalia | Animalia | Animalia | Animalia |
Beyond traditional kingdoms
While the concept of kingdoms continues to be used, and is recognized by international taxonomic committees, there are also other systems, including proposals for higher-level classifications, such as domain, which has wide acceptance. There is also a movement away from traditional kingdoms on the basis of their no longer being seen as providing a cladistic classification, where there is emphasis in arranging organisms into natural groups (Simpson and Roger 2004).
Three domains of life
Based on such RNA studies, Carl Woese thought life could be divided into three large divisions and referred to them as the "three primary kingdom" model or "urkingdom" model (Balch et al. 1977; Worse and Fox 1977). In 1990, the name "domain" was proposed for the highest rank (Worse et al. 1990). This term represents a synonym for the category of dominion (lat. dominium), introduced by Moore in 1974.
Woese divided the prokaryotes (previously classified as the Kingdom Monera) into two groups, called Eubacteria and Archaebacteria, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes.
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According to genetic data, although eukaryote groups such as plants, fungi, and animals may look different, they are more closely related to each other than they are to either the Eubacteria or Archaea. It was also found that the eukaryotes are more closely related to the Archaea than they are to the Eubacteria. Although the primacy of the Eubacteria-Archaea divide has been questioned, it has been upheld by subsequent research (Dagen et al. 2010).
Eukaryotic supergroups
In 2004, a review article by Simpson and Roger noted that the Protista were "a grab-bag for all eukaryotes that are not animals, plants, or fungi." They held that only monophyletic groups should be accepted as formal ranks in a classification and that—while this approach had been impractical previously (necessitating "literally dozens of eukaryotic "kingdoms"—it had now become possible to divide the eukaryotes into "just a few major groups that are probably all monophyletic" (Simpson and Roger 2004). .
On this basis, the diagram below (redrawn from their article) showed the real "kingdoms" (their quotation marks) of the eukaryotes (Simpson and Roger 2004). A classification which followed this approach was produced in 2005 for the International Society of Protistologists, by a committee which "worked in collaboration with specialists from many societies." It divided the eukaryotes into the same six "supergroups" (Adl et al. 2005). The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom."
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In this system the multicellular animals (Metazoa) are descended from the same ancestor as both the unicellular choanoflagellates and the fungi which form the Opisthokonta. Plants are thought to be more distantly related to animals and fungi.
However, in the same year as the International Society of Protistologists' classification was published (2005), doubts were being expressed as to whether some of these supergroups were monophyletic, particularly the Chromalveolata (Harper et al. 2005), and a review in 2006 noted the lack of evidence for several of the six proposed supergroups (Parfrey et al. 2006).
Viruses
The International Committee on Taxonomy of Viruses uses the taxonomic rank "kingdom" in the classification of viruses (with the suffix -virae), but this is beneath the top level classifications of realm and subrealm.
Non-cellular life, most notably the viruses, is not included in the traditional Linnaean taxonomic system. Van der Gulik et al. (2023) have proposed adding another level above domain, World (Latin alternative mundis) to include non-cellular entities (as well as the rank of Empire—Latin alternative imperium—to accommodate the uncovered diversity in unicellular eukaryotes). Under their proposal, there would be two groups within the taxonomic rank of World: a world of cells and a world of viruses. There proposal would result in a ten-rank system: species, genus, family, order, class, phylum, kingdom, empire, domain, and world.
See also
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Kingdom (biology) Portal |
Notes
- ↑ C. Linnaeus, Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species (Leiden: Haak, 1735).
- ↑ E. Haeckel, Generelle Morphologie der Organismen (Berlin: Reimer, 1866)
- ↑ E. Chatton, "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires," Annales des Sciences Naturelles - Zoologie et Biologie Animale 10-VIII (1925): 5–84.
- ↑ E. Chatton, Titres et Travaux Scientifiques (1906–1937) (Sète, France: E. Sottano, 1937).
- ↑ H. F. Copeland, "The kingdoms of organisms," Quarterly Review of Biology 13, issue 4(1938): 383–420.
- ↑ H. F. Copeland, The Classification of Lower Organisms (Palo Alto: Pacific Books, 1956).
- ↑ R. H. Whittaker, "New concepts of kingdoms of organisms," Science 163, issue 3863(1969): 150–160. PMID 5762760.
- ↑ C. R. Woese, W. E. Balch, L. J. Magrum, G. E. Fox, and R. S. Wolfe, "An ancient divergence among the bacteria," Journal of Molecular Evolution 9, issue 4(1977): 305–311. PMID 408502.
- ↑ C. R. Woese, and G. E. Fox. "Phylogenetic structure of the prokaryotic domain: the primary kingdoms," Proceedings of the National Academy of Sciences of the United States of America 74, issue 11(1977): 5088–5090. PMID 270744.
- ↑ C. Woese, O. Kandler, and M. Wheelis, "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya," Proceedings of the National Academy of Sciences of the United States of America 87, issue 12(1990): 4576–4579. PMID 2112744.
- ↑ T. Cavalier-Smith, "Eukaryote kingdoms: Seven or nine?" Bio Systems 14, issue 3–4(1991): 461–481. PMID 7337818.
- ↑ T. Cavalier-Smith, "Origins of secondary metabolism," Ciba Foundation Symposium (of series Novartis Foundation Symposia) 171(1992): 64–80. PMID 1302186
- ↑ T. Cavalier-Smith, "Kingdom protozoa and its 18 phyla," Microbiological Reviews 57, issue 4(1993): 953–994. PMID 8302218.
- ↑ T. Cavalier-Smith, "A revised six-kingdom system of life," Biological Reviews 73(1998): 203–66. PMID 9809012.
- ↑ T. Cavalier-Smith, "Only six kingdoms of life, Proceedings of the Royal Society of London B 271, issue 1545(2004): 1251–1262. PMID 15306349.
- ↑ T. Cavalier-Smith, Thomas. "Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree, Biology Letters 6 issue 3(2010): 342–345. PMID 20031978.
- ↑ M. A. Ruggiero, D. P. Gordon, T. M. Orrell, N. Bailly, T. Bourgoin, R. C. Brusca, T. Cavalier-Smith, et al., "A higher level classification of all living organisms," PLOS ONE 10, issue 4(2015): PMID 25923521
ReferencesISBN links support NWE through referral fees
- Adl, S. M., M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, et al. 2005. The new higher-level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology 52(5): 399–451. PMID 16248873.
- Balch, W. E., L. J. Magrum, G. E. Fox, R. S. Wolfe, and C. R. Woese. 1977. An ancient divergence among the bacteria. Journal of Molecular Evolution 9(4): 305–311. PMID 408502.
- Case, Emily. 2008. Teaching taxonomy: how many kingdoms?. American Biology Teacher 70(8): 472–477.
- Cavalier-Smith, Thomas. 1987. Eucaryotes with no mitochondria. Nature 326(6111): 332–333. PMID 326332a0.
- Cavalier-Smith, Thomas. 1991. Eukaryote kingdoms: Seven or nine? Bio Systems 14(3–4): 461–481. PMID 7337818.
- Cavalier-Smith, Thomas. 1992. Origins of secondary metabolism. Ciba Foundation Symposium (of series Novartis Foundation Symposia) 171(1992): 64–80. PMID 1302186
- Cavalier-Smith, Thomas. 1993. Kingdom protozoa and its 18 phyla. Microbiological Reviews 57(4): 953–994. PMID 8302218.
- Cavalier-Smith, T. 1998. A revised six-kingdom system of life. Biological Reviews 73: 203–66. PMID 9809012.
- Cavalier-Smith, T. 2004. Only six kingdoms of life. Proceedings of the Royal Society of London B 271(1545): 1251–1262. PMID 15306349.
- Cavalier-Smith, Thomas. 2010. Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree. Biology Letters 6(3): 342–345. PMID 20031978.
- Chatton, Édouard. 1925. Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires. Annales des Sciences Naturelles - Zoologie et Biologie Animale 10-VIII: 5–84.
- Chatton, Édouard. 1937. Titres et Travaux Scientifiques (1906–1937). Sète, France: E. Sottano.
- Copeland, Herbert. 1938. The kingdoms of organisms. Quarterly Review of Biology 13(4): 383–420.
- Copeland, Herbert. 1956. The Classification of Lower Organisms. Palo Alto: Pacific Books.
- Cracraft, Joel, and Michael J. Donaghue. 2004. Assembling the Tree of Life. Oxford University Press. ISBN 0195172345.
- Dagan, T., M. Roettger, D. Bryant, and W. Martin. 2010. Genome networks root the tree of life between prokaryotic domains. Genome Biology and Evolution 2: 379–92. PMID 20624742.
- Darwin, C. 1859. On the Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London: John Murray, Albemarle Street. Reprinted: Gramercy, 1995.
- Haeckel, Ernst. 1866. Generelle Morphologie der Organismen. Berlin: Reimer.
- Harper, J. T., E. Waanders, and P. J. Keeling. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. International Journal of Systematic and Evolutionary Microbiology 55(Pt.1). PMID 15653923.
- Linnaeus, Carl. 1735. Systema Naturae: sive regna tria naturae systematice proposita per classes, ordines, genera, & species. Leiden, Haak.
- Linnaeus, Carl. 1753. Species Plantarum. Stockholm.
- Linnaeus, Carl. 1758. Systema Naturae: sive regna tria naturae systematice proposita per classes, ordines, genera, & species, 10th edition. Leiden: Haak.
- Margulis, L., and M. J. Chapman. 2009. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. Academic Press. ISBN 9780080920146.
- Mayr, Ernst. 1968. The role of systematics in biology: The study of all aspects of the diversity of life is one of the most important concerns in biology. Science 159(3815): 595–599. PMID 4886900.
- Mayr, Ernst. 1982. The Growth of Biological Thought. Cambridge, MA: Belknap Press of Harvard University Press.
- Mayr, E. 1998. Two empires or three? Proceedings of the National Academy of Sciences 95(17): 9720-9723.
- McDonald, David. 2008. Molecular marker glossary. ‘’University of Wyoming’’. Retrieved June 30, 2024.
- Moore, R. T. 1974. Proposal for the recognition of super ranks. Taxon 23(4): 650–652.
- Michener, Charles D., John O. Corliss, Richard S. Cowan, Peter H. Raven, Curtis W. Sabrosky, Donald S. Squires, and G. W. Wharton. 1970. Systematics In Support of Biological Research. Washington, D.C.: Division of Biology and Agriculture, National Research Council.
- Palmer, Laurence, and Seymour H. Fowler. 1975. Fieldbook of Natural History, 2nd Edition. McGraw-Hill. ISBN 978-0-070-48425-2.
- Parfrey, L. W., E. Barbero, E. Lasser, M. Dunthorn, et al. 2006. Evaluating support for the current classification of eukaryotic diversity. PLOS Genetics 2(12): e220. PMID 17194223.
- Poole, Anthony, and David Penny. 2007. Engulfed by speculation. Nature 447(7147): 913. PMID 17581566.
- Ruggiero, Michael A., Dennis P. Gordon, Thomas M. Orrell, Nicolas Bailly, Thierry Bourgoin, Richard C. Brusca, Thomas Cavalier-Smith, et al. 2015. A higher level classification of all living organisms. PLOS ONE 10(4): e0119248. PMID 25923521.
- Sapp, J. 2005. The prokaryote-eukaryote dichotomy: meanings and mythology. Microbiology and Molecular Biology Reviews 69(2): 292–305. PMID 15944457.
- Scamardella, Joseph M. 1999. Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista. International Microbiology 2(4): 207–16. PMID 10943416.
- Simpson, Alastair G. B., and Andrew J. Roger. 2004. The real 'kingdoms' of eukaryotes. Current Biology 14(17): R693–R696. PMID 15341755.
- Singer, Charles J. (1931). A Short History of Biology: A General Introduction to the Study of Living Things. Oxford: Clarendon Press.
- van der Gulik, Peter T. S., Wouter D. Hoff, and David Speijer. 2023. Reviewing Linnaean taxonomy: a proposal to restructure the highest levels of the Natural System. Biol. Rev. 98: 384-602.
- Whittaker, R. H. 1969. New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms. Science 163(3863): 150–60. PMID 5762760.
- Woese, C. R. 1998. Default taxonomy: Ernst Mayr's view of the microbial world. Proceedings of the National Academy of Sciences 95(19): 11043-11046.
- Worse, C. R., and G. E. Fox. 1977. Phylogenetic structure of the prokaryote domain: the primary kingdoms. Proc. Natl. Acad. Sci. 74(11): 5088-5090.
- Worse, C. R., W. R. Balch, L. J. Magrum, G. E. Fox, and R. S. Wolfe. 1977. An ancient divergence among the bacteria. Journal of Molecular Evolution 9(4): 305–311. PMID 408502.
- Woese, C. R., O. Kandler, and M. I. Wheelis. 1990. Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the Natural Academy of Sciences of the United States of America 87(12): 4576, 4579.
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