Inbiology, akingdom is the second highesttaxonomic rank, just belowdomain. Kingdoms are divided into smaller groups calledphyla (singular phylum).
Traditionally, textbooks from the United States and some of Canada have used a system ofsix kingdoms (Animalia,Plantae,Fungi,Protista,Archaea/Archaebacteria, andBacteria or Eubacteria), while textbooks in other parts of the world, such as Bangladesh, Brazil, Greece, India, Pakistan, Spain, and the United Kingdom have usedfive kingdoms (Animalia, Plantae, Fungi, Protista andMonera).
Some recent classifications based on moderncladistics have explicitly abandoned the termkingdom, noting that some traditional kingdoms are notmonophyletic, meaning that they do not consist of all thedescendants of a commonancestor. The termsflora (for plants),fauna (for animals), and, in the 21st century,funga (for fungi) are also used for life present in a particular region or time.[1][2]
Prefixes can be added sosubkingdom (subregnum) andinfrakingdom (also known asinfraregnum) 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 rankbranch (Latin:ramus) can be inserted between subkingdom and infrakingdom, e.g.,Protostomia andDeuterostomia in the classification of Cavalier-Smith.[6]
Haeckel's original (1866) conception of the three kingdoms of life, including the new kingdom Protista. Notice the inclusion of the cyanobacteriumNostoc with plants.
In 1674,Antonie van Leeuwenhoek, often called the "father of microscopy", sent theRoyal 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".[8]
In 1860John Hogg proposed theProtoctista, 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.[8] In 1866,Ernst Haeckel also proposed a third kingdom of life, theProtista, 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.[8] 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).[8]
The development ofmicroscopy revealed important distinctions between those organisms whose cells do not have a distinctnucleus (prokaryotes) and organisms whose cells do have a distinct nucleus (eukaryotes). In 1937Édouard Chatton introduced the terms "prokaryote" and "eukaryote" to differentiate these organisms.[9]
In 1938,Herbert F. Copeland proposed a four-kingdom classification by creating the novel KingdomMonera of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified asBacteria andArchaea. Ernst Haeckel, in his 1904 bookThe 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 phylumCyanobacteria.[8][9]
In the 1960s,Roger Stanier andC. 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—asuperkingdom orempire—with thetwo-empire system of prokaryotes and eukaryotes.[9] The two-empire system would later be expanded to thethree-domain system of Archaea, Bacteria, and Eukaryota.[10]
The differences betweenfungi 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,[8] but was largely ignored in this separation by scientists of his time.Robert Whittaker recognized an additional kingdom for theFungi.[11] 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 upon differences innutrition; his Plantae were mostly multicellularautotrophs, his Animalia multicellularheterotrophs, and his Fungi multicellularsaprotrophs.
The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies.[11] The five kingdom system may be combined with the two empire system. In the Whittaker system, Plantae included some algae. In other systems, such asLynn Margulis's system of five kingdoms, the plants included just the land plants (Embryophyta), and Protoctista has a broader definition.[12]
Following publication of Whittaker's system, the five-kingdom model began to be commonly used in high school biology textbooks.[13] 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).[14]
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 onribosomal RNA structure;[15] this would later lead to the proposal ofthree "domains" of life, of Bacteria, Archaea, and Eukaryota.[5] Combined with the five-kingdom model, this created a six-kingdom model, where the kingdom Monera is replaced by the kingdoms Bacteria and Archaea.[16] This six-kingdom model is commonly used in recent US high school biology textbooks, but has received criticism for compromising the current scientific consensus.[13] But the division of prokaryotes into two kingdoms remains in use with the recentseven kingdoms scheme of Thomas Cavalier-Smith, although it primarily differs in that Protista is replaced byProtozoa andChromista.[17]
Thomas Cavalier-Smith supported the consensus at that time, that the difference betweenEubacteria andArchaebacteria was so great (particularly considering the genetic distance of ribosomal genes) that the prokaryotes needed to be separated into two different kingdoms. He then dividedEubacteria into two subkingdoms:Negibacteria (Gram-negative bacteria) andPosibacteria (Gram-positive bacteria). Technological advances in electron microscopy allowed the separation of theChromista from thePlantae kingdom. Indeed, the chloroplast of the chromists is located in the lumen of theendoplasmic reticulum instead of in thecytosol. Moreover, only chromists containchlorophyll 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.[18] As mitochondria were known to be the result of theendosymbiosis of aproteobacterium, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step ineukaryogenesis. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdomArchezoa. This superkingdom was opposed to theMetakaryota superkingdom, grouping together the five other eukaryotic kingdoms (Animalia,Protozoa,Fungi,Plantae andChromista). This was known as theArchezoa hypothesis, which has since been abandoned;[19] later schemes did not include the Archezoa–Metakaryota divide.[6][17]
In 1998, Cavalier-Smith published a six-kingdom model,[6] which has been revised in subsequent papers. The version published in 2009 is shown below.[20][a][21] Cavalier-Smith no longer accepted the importance of the fundamental Eubacteria–Archaebacteria divide put forward by Woese and others and supported by recent research.[22] The kingdomBacteria (sole kingdom of empireProkaryota) was subdivided into two sub-kingdoms according to their membrane topologies:Unibacteria andNegibacteria. Unibacteria was divided into phylaArchaebacteria andPosibacteria; 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 bemonophyletic ("holophyletic" in his terminology) to be valid. He defines Prokaryota, Bacteria, Negibacteria, Unibacteria, and Posibacteria as validparaphyla (therefore "monophyletic" in the sense he uses this term) taxa, marking important innovations of biological significance (in regard of the concept of biologicalniche).
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 bearchezoans (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 factmetakaryotes, according to the significance of the term given by Cavalier-Smith. Some of the members of the defunct kingdomArchezoa, like the phylumMicrosporidia, were reclassified into kingdomFungi. Others were reclassified in kingdomProtozoa, likeMetamonada which is now part of infrakingdomExcavata.
Because Cavalier-Smith allowsparaphyly, the diagram below is an "organization chart", not an "ancestor chart", and does not represent an evolutionary tree.
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 andArchaea. (This was based on the consensus in theTaxonomic Outline of Bacteria and Archaea, and theCatalogue of Life). The Eukaryota have five kingdoms: Protozoa, Chromista, Plantae, Fungi, and Animalia. In this classification aprotist is any of the eukaryoticunicellular organisms.[17]
The kingdom-level classification of life is still widely employed as a useful way of grouping organisms, notwithstanding some problems with this approach:
The most recent research does not support the classification of the eukaryotes into any of the standard systems. 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."[40] Kingdoms are rarely used in academic phylogeny and are more common in introductory education, where 5–6 kingdom models are preferred.[41]
While the concept of kingdoms continues to be used by some taxonomists, there has been a movement away from traditional kingdoms, as they are no longer seen as providing acladistic classification, where there is emphasis in arranging organisms intonatural groups.[42]
Based on 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.[15]
In 1990, the name "domain" was proposed for the highest rank, with Latin equivalentregio.[5] Woese divided the prokaryotes (previously classified as the Kingdom Monera) into two groups, calledEubacteria andArchaebacteria, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes.
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.[22] There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.
In 2004, a review article by Simpson and Roger noted that the Protista were "agrab-bag for alleukaryotes 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".[42]
On this basis, the diagram opposite (redrawn from their article) showed the real "kingdoms" (their quotation marks) of the eukaryotes.[42] 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".[43] The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom".
One hypothesis of eukaryotic relationships depicted by Alastair Simpson
In this system the multicellular animals (Metazoa) are descended from the same ancestor as both the unicellularchoanoflagellates and the fungi which form theOpisthokonta.[43] 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,[44] and a review in 2006 noted the lack of evidence for several of the six proposed supergroups.[45]
As of 2019[update], there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in aclade dubbed theSAR supergroup,[46] so that Rhizaria is not one of the main eukaryote groups.[20][47][48][49][50]
Some authors have addednon-cellular life to their classifications. This can create a "superdomain" called "Acytota", also called "Aphanobionta", of non-cellular life; with the other superdomain being "cytota" or cellular life.[52][53] (see section below for further discussion)
There is ongoing debate as to whetherviruses can be included in the tree of life. The arguments against include the fact that they are obligate intracellularparasites that lackmetabolism and are not capable ofreplication outside of a host cell.[58][59] Another argument is that their placement in the tree would be problematic, since it is suspected that viruses have various evolutionary origins,[58] and they have a penchant for harvestingnucleotide sequences from their hosts.
^"IUCN SSC acceptance of Fauna Flora Funga"(PDF). Fungal Conservation Committee, IUCN SSC. 2021. Archived fromthe original(PDF) on 2021-11-11. Retrieved2022-03-04.The IUCN Species Survival Commission calls for the due recognition of fungi as major components of biodiversity in legislation and policy. It fully endorses the Fauna Flora Funga Initiative and asks that the phrasesanimals and plants andfauna and flora be replaced withanimals, fungi, and plants andfauna, flora, and funga.
^Singer, Charles J. (1931).A short history of biology, a general introduction to the study of living things. Oxford: Clarendon Press.OCLC1197036.
^abcdefScamardella, 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.PMID10943416.
