In biology, kingdom (Latin: regnum, pl. regna) is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla (in zoology) or divisions in botany.
Currently, textbooks from the United States use a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea, and Bacteria) while British, Australian and Latin American textbooks may describe five kingdoms (Animalia, Plantae, Fungi, Protoctista, and Prokaryota or Monera). Some recent classifications have explicitly abandoned the term "kingdom", noting that the traditional kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common ancestor.
Definition and associated terms 
When Carl Linnaeus introduced the rank-based system of nomenclature into biology, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order, family, genus and species. In the 1960s a rank was introduced above kingdom, namely domain (or empire), so that kingdom is no longer the highest rank.
Prefixes can be added to subkingdom and infrakingdom 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). Because of its position, branch can be considered as a minor rank of the kingdom group even if it is not etymologically derived from it.
Systems of classification 
Historically, the number of kingdoms in widely accepted classifications has grown from two to six. However, phylogenetic research from about 2000 onwards does not support any of the traditional systems.
An initial dichotomy: Two kingdoms 
The classification of living things into animals and plants is an ancient one. Aristotle (384–322 BC) classified animal species in his work The History of Animals, and his pupil Theophrastus (c. 371–c. 287 BC) wrote a parallel work on plants (Historia Plantarum (The History of Plants)).
Carolus Linnaeus (1707–1778) laid the foundations for modern biological nomenclature, now regulated by the Nomenclature Codes. He distinguished two kingdoms of living things: Regnum Animale ('animal kingdom') for animals and Regnum Vegetabile ('vegetable kingdom') for plants. (Linnaeus also included minerals, placing them in a third kingdom, Regnum Lapideum.) Linnaeus divided each kingdom into classes, later grouped into phyla for animals and divisions for plants.
An increasing number of kingdoms 
Three kingdoms 
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. At first, these organisms were divided into animals and plants and placed in the appropriate Kingdom. However, by the mid-19th century, it had become clear that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded". In 1866, following earlier proposals by Richard Owen and John Hogg, Ernst Haeckel proposed a third kingdom of life. 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).
Four kingdoms 
The development of microscopy, and the electron microscope in particular, revealed an important distinction between those unicellular organisms whose cells do not have a distinct nucleus, prokaryotes, and those unicellular and multicellular organisms whose cells do have a distinct nucleus, eukaryotes. In 1938, Herbert F. Copeland proposed a four-kingdom classification, moving the two prokaryotic groups, bacteria and "blue-green algae", into a separate Kingdom Monera.
The importance of the distinction between prokaryotes and eukaryotes gradually became apparent. In the 1960s Stanier and van Niel popularized Édouard Chatton's much earlier proposal to recognize this division in a formal classification. This required the creation, for the first time, of a rank above kingdom, a superkingdom or empire, also called a domain.
Five kingdoms 
The differences between fungi and other organisms regarded as plants had long been recognized. For example, at one point Haeckel moved the fungi out of Plantae into Protista, before changing his mind. Robert Whittaker recognized an additional kingdom for the Fungi. The resulting five-kingdom system, proposed in 1969 by Whittaker, has become 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 on 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. The five kingdom system may be combined with the two empire system.
Recent developments: six kingdoms or more? 
The three domains of life 
From around the mid-1970s onwards, there was an increasing emphasis on comparisons of genes on the molecular level (initially ribosomal RNA genes) as the primary factor in classification; genetic similarity was stressed over outward appearances and behavior. Taxonomic ranks, including kingdoms, were to be groups of organisms with a common ancestor, whether monophyletic (all descendants of a common ancestor) or paraphyletic (only some descendants of a common ancestor).
Based on such RNA studies, Carl Woese divided the prokaryotes (hitherto 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. Similarly, though eukaryote groups such as plants, fungi and animals may look different, they are more closely related to each other from a genetic standpoint than they are to either the Eubacteria or Archaebacteria. It was also found that the eukaryotes are more closely related, genetically, to the Archaebacteria than they are to the Eubacteria.
Although the primacy of the eubacteria-archaebacteria divide has been questioned, it has been upheld by subsequent research.
Woese attempted to establish a "three primary kingdom" or "urkingdom" system. In 1990, the name "domain" was proposed for the highest rank. The six-kingdom system shown below represents a blending of the classic five-kingdom system and Woese's three-domain system. Such six-kingdom systems have become standard in many works.
Woese also recognized that the kingdom Protista was not a monophyletic group and might be further divided at the level of kingdom.
Cavalier-Smith's system 
Thomas Cavalier-Smith has published extensively on the evolution and classification of life, particularly protists. His views have been influential but controversial, and not always widely accepted.
Eight kingdoms 
Thomas Cavalier-Smith thought at first, as it was nearly consensually admitted at that time, that the difference between 'eubacteria' and 'archaebacteria' was so great (particularly considering the genetic distance of ribosomal genes) that they needed to be separated in two different kingdoms, hence splitting the empire Bacteria into two kingdoms. Eubacteria was divided into two subkingdoms: Negibacteria (Gram negative bacteria) and Posibacteria (Gram positive bacteria).
Technological advances in electronical 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 do 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. As mitochondrion was 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 was known as the Archezoa hypothesis. This superkingdom was opposed to the Metakaryota superkingdom, grouping together the five other eukaryotic kingdoms (Animalia, Protozoa, Fungi, Plantae and Chromista).
Six kingdoms 
In 1998, he published a six-kingdom model, which has been revised in subsequent papers. The version published in 2009 is shown below. (Compared to the version he published in 2004, the alveolates and the rhizarians have been moved from Kingdom Protozoa to Kingdom Chromista.) Cavalier-Smith does not accept anymore the importance of the fundamental eubacteria–archaebacteria divide put forward by Woese and others and supported by recent research. His Kingdom Bacteria includes Archaebacteria as a phylum of the subkingdom Unibacteria which comprises only one other phylum: the Posibacteria. The two subkingdoms Unibacteria and Negibacteria of kingdom Bacteria (sole kingdom of empire Prokaryota) are opposed according to their membrane topologies. The bimembranous-unimembranous transition is 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 paraphyletic (therefore "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 to realize that all the phyla thought to be archezoans (i.e. primitively amitochondriate eukaryotes) had in fact secondarily lost their mitochondria, most of the time 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.
The diagram below does not represent an evolutionary tree.
International Society of Protistologists Classification (2005) 
The "classic" six-kingdom system is still recognizably a modification of the original two-kingdom system: Animalia remains; the original category of plants has been split into Plantae and Fungi; and single-celled organisms have been introduced and split into Bacteria, Archaea and Protista.
Research published in the 21st century has produced a rather different picture. 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". On this basis, the diagram opposite (redrawn from their article) showed the real 'kingdoms' (their quotation marks) of the eukaryotes. 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". The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom".
In this system, the traditional kingdoms have vanished. For example, research shows that the multicellular animals (Metazoa) are descended from the same ancestor as the unicellular choanoflagellates and the fungi. A classification system which places these three groups into different kingdoms (with multicellular animals forming Animalia, choanoflagellates part of Protista and Fungi a separate kingdom) is not monophyletic. The monophyletic group is the Opisthokonta, made up of all those organisms believed to have descended from a common ancestor, some of which are unicellular (choanoflagellates), some of which are multicellular but not closely related to animals (some fungi), and others of which are traditional multicellular animals.
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, and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.
As of 2010[update], there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a clade dubbed the SAR supergroup, so that Rhizaria is not one of the main eukaryote groups. Beyond this, there does not appear to be a consensus. Rogozin et al. in 2009 noted that "The deep phylogeny of eukaryotes is an extremely difficult and controversial problem." As of December 2010[update], there appears to be a consensus that the 2005 six supergroup model does not reflect the true phylogeny of the eukaryotes and hence how they should be classified, although there is no agreement as to the model which should replace it.
The sequence from the two-kingdom system up to Cavalier-Smith's six-kingdom system can be summarized in the table below.
|Woese et al.
|Woese et al.
|2 kingdoms||3 kingdoms||2 empires||4 kingdoms||5 kingdoms||6 kingdoms||3 domains||8 kingdoms||6 kingdoms|
Note that the equivalences in this table are not perfect. For example, Haeckel placed the red algae (his Florideae, modern Florideophyceae) and blue-green algae (his Archephyta, modern Cyanobacteria) in his Plantae.
The kingdom-level classification of life is still widely employed as a useful way of grouping organisms, notwithstanding some problems with this approach:
- Kingdoms such as Bacteria represent grades rather than clades, and so are rejected by phylogenetic classification systems.
- The most recent research does not support the classification of the eukaryotes into any of the standard systems. As of April 2010[update], no set of kingdoms is sufficiently supported by research to attain widespread acceptance. In 2009, Andrew Roger and Alastair Simpson emphasized the need for diligence in analyzing new discoveries: "With the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."
See also 
- Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
- See e.g. McNeill, J.; Barrie, F. R.; Burdet, H. M. et al., eds. (2006), International Code of Botanical Nomenclature (Vienna Code) Adopted by the Seventeenth International Botanical Congress, Vienna, Austria, July 2005 (electronic ed.), Vienna: International Association for Plant Taxonomy, retrieved 2011-02-20 , article 3.1
- Cavalier-Smith, T. (1998), "A revised six-kingdom system of life", Biological Reviews 73 (03): 203–66, doi:10.1111/j.1469-185X.1998.tb00030.x, PMID 9809012
- Singer, Charles J. (1931), A short history of biology, a general introduction to the study of living things, Oxford: Clarendon Press, OCLC 1197036
- 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
- Stanier, R.Y. & Van Neil, C.B. (1962), "The concept of a bacterium", Archiv für Mikrobiologie 42 (1): 17–35, doi:10.1007/BF00425185, PMID 13916221
- Whittaker, R.H. (January 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, Bibcode:1969Sci...163..150W, doi:10.1126/science.163.3863.150, PMID 5762760
- Dagan, T.; Roettger, M.; Bryant & Martin, W. (2010), "Genome Networks Root the Tree of Life between Prokaryotic Domains", Genome Biology and Evolution 2: (0): 379–92, doi:10.1093/gbe/evq025
- Balch, W.E.; Magrum, L.J.; Fox, G.E.; Wolfe, C.R.; & Woese, C.R. (August 1977), "An ancient divergence among the bacteria", J. Mol. Evol. 9 (4): 305–11, doi:10.1007/BF01796092, PMID 408502
- Woese, C.R.; Kandler, O. & Wheelis, M. (1990), "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya", Proc Natl Acad Sci U S A 87 (12): 4576–9, Bibcode:1990PNAS...87.4576W, doi:10.1073/pnas.87.12.4576, PMC 54159, PMID 2112744
- M. Rivera & J. Lake 2004, The ring of life provides evidence for a genome fusion origin of eukaryotes Nature 431, 152-155
- Palaeos.com, Origins of the Eukarya, archived from the original on 2010-04-29, retrieved 2010-04-29
- Cavalier-Smith, Thomas (2009), "Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree", Biology Letters 6 (3): 342–5, doi:10.1098/rsbl.2009.0948, PMC 2880060, PMID 20031978
- Cavalier-Smith, T. (2004), "Only six kingdoms of life", Proc. R. Soc. Lond. B 271 (1545): 1251–62, doi:10.1098/rspb.2004.2705, PMC 1691724, PMID 15306349, retrieved 2010-04-29
- Simpson, Alastair G.B. & Roger, Andrew J. (2004), "The real ‘kingdoms’ of eukaryotes", Current Biology 14 (17): R693–6, doi:10.1016/j.cub.2004.08.038, PMID 15341755
- Adl, Sina M.; et al. (2005), "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists", Journal of Eukaryotic Microbiology 52 (5): 399, doi:10.1111/j.1550-7408.2005.00053.x, PMID 16248873
- Harper, J.T.; Waanders, E. & Keeling, P. J. (2005), "On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes", Nt. J. System. Evol. Microbiol. 55 (Pt 1): 487–496, doi:10.1099/ijs.0.63216-0, PMID 15653923
- Parfrey, Laura W.; Barbero, Erika; Lasser, Elyse; Dunthorn, Micah; Bhattacharya, Debashish; Patterson, David J. & Katz, Laura A. (2006), "Evaluating Support for the Current Classification of Eukaryotic Diversity", PLoS Genet. 2 (12): e220, doi:10.1371/journal.pgen.0020220, PMC 1713255, PMID 17194223
- Burki et al. 2007, p. 4
- Burki, Fabien; Shalchian-Tabrizi, Kamran; Minge, Marianne; Skjæveland, Åsmund; Nikolaev, Sergey I.; Jakobsen, Kjetill S. & Pawlowski, Jan (2007), "Phylogenomics Reshuffles the Eukaryotic Supergroups", in Butler, Geraldine, PLoS ONE 2 (8): e790, Bibcode:2007PLoSO...2..790B, doi:10.1371/journal.pone.0000790, PMC 1949142, PMID 17726520
- Burki, Fabien; Shalchian-Tabrizi, Kamran & Pawlowski, Jan (2008), "Phylogenomics reveals a new 'megagroup' including most photosynthetic eukaryotes", Biology Letters 4 (4): 366–369, doi:10.1098/rsbl.2008.0224, PMC 2610160, PMID 18522922.
- Burki, F. et al.; Inagaki, Y.; Brate, J.; Archibald, J. M.; Keeling, P. J.; Cavalier-Smith, T.; Sakaguchi, M.; Hashimoto, T. et al. (2009), "Large-Scale Phylogenomic Analyses Reveal That Two Enigmatic Protist Lineages, Telonemia and Centroheliozoa, Are Related to Photosynthetic Chromalveolates", Genome Biology and Evolution 1 (0): 231–8, doi:10.1093/gbe/evp022, PMC 2817417, PMID 20333193
- Hackett, J.D.; Yoon, H.S.; Li, S.; Reyes-Prieto, A.; Rummele, S.E. & Bhattacharya, D. (2007), "Phylogenomic analysis supports the monophyly of cryptophytes and haptophytes and the association of Rhizaria with chromalveolates", Mol. Biol. Evol. 24 (8): 1702–13, doi:10.1093/molbev/msm089, PMID 17488740
- Rogozin, I.B.; Basu, M.K.; Csürös, M. & Koonin, E.V. (2009), "Analysis of Rare Genomic Changes Does Not Support the Unikont–Bikont Phylogeny and Suggests Cyanobacterial Symbiosis as the Point of Primary Radiation of Eukaryotes", Genome Biology and Evolution 1 (0): 99–113, doi:10.1093/gbe/evp011, PMC 2817406, PMID 20333181
- Kim, E.; Graham, L.E. & Redfield, Rosemary Jeanne (2008), "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata", in Redfield, Rosemary Jeanne, PLoS ONE 3 (7): e2621, Bibcode:2008PLoSO...3.2621K, doi:10.1371/journal.pone.0002621, PMC 2440802, PMID 18612431
- Haeckel, E. (1866). Generelle Morphologie der Organismen. Reimer, Berlin.
- Chatton, É. (1925). "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires". Annales des Sciences Naturelles - Zoologie et Biologie Animale. 10-VII: 1–84.
- Chatton, É. (1937). Titres et Travaux Scientifiques (1906–1937). Sette, Sottano, Italy.
- Copeland, H. (1938). "The kingdoms of organisms". Quarterly Review of Biology 13: 383–420. doi:10.1086/394568.
- Copeland, H. F. (1956). The Classification of Lower Organisms. Palo Alto: Pacific Books. doi:10.5962/bhl.title.4474.
- Woese, C. R.; Fox, G. E. (November 1977). "Phylogenetic structure of the prokaryotic domain: the primary kingdoms". Proceedings of the National Academy of Sciences of the United States of America 74 (11): 5088–90. doi:10.1073/pnas.74.11.5088. PMC 432104. PMID 270744.
- Woese, C.; Kandler, O.; Wheelis, M. (1990). "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 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
- Cavalier-Smith, T. (1981). "Eukaryote kingdoms: seven or nine?". Bio Systems 14 (3–4): 461–481. doi:10.1016/0303-2647(81)90050-2. PMID 7337818.
- Cavalier-Smith, T. (1992). "Origins of secondary metabolism". Ciba Foundation symposium 171: 64–80; discussion 80–7. PMID 1302186.
- Cavalier-Smith, T. (1993). "Kingdom protozoa and its 18 phyla". Microbiological reviews 57 (4): 953–994. PMC 372943. PMID 8302218.
- Cavalier-Smith, T. (1998), "A revised six-kingdom system of life", Biological Reviews 73 (03): 203–66, doi:10.1111/j.1469-185X.1998.tb00030.x, PMID 9809012
- Cavalier-Smith T (June 2010). "Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree". Biol. Lett. 6 (3): 342–5. doi:10.1098/rsbl.2009.0948. PMC 2880060. PMID 20031978.
- Roger, A.J. & Simpson, A.G.B. (2009), "Evolution: Revisiting the Root of the Eukaryote Tree", Current Biology 19 (4): R165–7, doi:10.1016/j.cub.2008.12.032, PMID 19243692