Kingdom (biology): Difference between revisions
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{{Biological classification}} |
{{Biological classification}} |
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In [[biology]], '''kingdom''' or '''regnum''' is a [[taxonomic rank]], which is either |
In [[biology]], '''kingdom''' or '''regnum''' is a [[taxonomic rank]], which is either the highest rank or in the more recent [[three-domain system]], the [[Rank (zoology)|rank]] below [[domain (biology)|domain]]. Kingdoms are divided into smaller groups called [[Phylum|phyla]] (in zoology) or divisions in botany. The complete sequence of ranks is [[life]], [[domain (biology)|domain]], kingdom, [[phylum]], [[class (biology)|class]], [[order (biology)|order]], [[family (biology)|family]], [[genus]], and [[species]]. |
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Currently, many textbooks from the United States use a system of six kingdoms ([[Animal]]ia, [[Plantae]], [[Fungi]], [[Protista]], [[Archaea]], [[Bacteria]]) while British and Australian textbooks may describe five kingdoms ([[Animalia]], [[Plantae]], [[Fungi]], [[Protista]], and [[Prokaryota]] or [[Monera]]). |
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==Early concepts== |
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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. |
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==Two kingdoms== |
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The classification of living things into animals and plants is an ancient one. Thus [[Aristotle]] (384 BC – 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 (the [[Historia Plantarum|History of Plants]]).<ref>{{Citation |last=Singer |first=Charles J. |year=1931 |title=A short history of biology, a general introduction to the study of living things |publication-place=Oxford |publisher=Clarendon Press |oclc=1197036 }}</ref> |
The classification of living things into animals and plants is an ancient one. Thus [[Aristotle]] (384 BC – 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 (the [[Historia Plantarum|History of Plants]]).<ref>{{Citation |last=Singer |first=Charles J. |year=1931 |title=A short history of biology, a general introduction to the study of living things |publication-place=Oxford |publisher=Clarendon Press |oclc=1197036 }}</ref> |
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[[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 [[plant]]s. (Linnaeus also included [[mineral]]s, placing them in a third kingdom, ''[[Mineralia|Regnum Lapideum]]''.) Linnaeus divided each kingdom into classes, later grouped into [[phylum|phyla]] for animals and [[Division (biology)|divisions]] for 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 [[plant]]s. (Linnaeus also included [[mineral]]s, placing them in a third kingdom, ''[[Mineralia|Regnum Lapideum]]''.) Linnaeus divided each kingdom into classes, later grouped into [[phylum|phyla]] for animals and [[Division (biology)|divisions]] for plants. |
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{{clade |
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It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized [[Édouard Chatton]]'s proposal in the 1960s to divide them.<ref name="Stanier">{{cite journal |author=Stanier RY, Van Niel CB |title=The concept of a bacterium |journal=Archiv Für Mikrobiologie |volume=42 |issue= |pages=17–35 |year=1962 |pmid=13916221 }}</ref> |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1=Regnum Vegetabile |
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|2=Regnum Animale |
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}} |
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}} |
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}} |
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==Three kingdoms== |
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[[Cladistics]] does not use this term, because one of the fundamental premises of cladistics is that the evolutionary tree is so deep and so complex that it is inadvisable to set a fixed number of levels. |
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In 1674, [[Antonie van Leeuwenhoek]], often called the "father of microscopy", had sent the [[Royal Society]] of London a copy of his first observations of microscopic single-celled organisms. Up to this time, 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-1800s it had become clear that "the existing dichotomy of the plant and animal kingdoms [had] became rapidly blurred at its boundaries and outmoded".<ref name=Scamardella1999>{{Citation |last=Scamardella |first=Joseph M. |year=1999 |title=Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista |journal=International Microbiology |volume=2 |pages=207–16 }}</ref>. 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).<ref name=Scamardella1999/> |
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{{clade |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1=Kingdom Protista |
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|2=Kingdom Plantae |
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|3=Kingdom Animalia |
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}} |
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}} |
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}} |
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==Four kingdoms== |
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The development of microscopy, and the [[electron microscope]] in particular revealed an important distinction between those unicellular organisms whose cells did not have a distinct [[nucleus]], [[prokaryote]]s, and those unicellular and multicellular organisms whose cells did have a distinct nucleus, [[eukaryote]]s. In 1938, [[Herbert Copeland|Herbert F. Copeland]] proposed a four-kingdom classification, moving the two prokaryotic groups, bacteria and "blue-green algae" into a separate Kingdom Monera.<ref name=Scamardella1999/> |
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{{clade |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1=Kingdom Monera (prokaryotes, i.e. bacteria and "blue-green algae") |
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|2=Kingdom Protista (single-celled eukaryotes) |
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|3=Kingdom Plantae |
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|4=Kingdom Animalia |
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}} |
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}} |
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}} |
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It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized [[Édouard Chatton]]'s proposal in the 1960s to recognize this division in a formal classification. This required the creation, for the first time, of a rank above kingdom, superkingdom or empire.<ref name="Stanier">{{Citation |last=Stanier |first=R.Y. |last2=Van Neil |first2=C.B. |year=1962 |title=The concept of a bacterium |journal=Archiv Für Mikrobiologie |volume=42 |pages=17–35 |pmid=13916221 |lastauthoramp=yes }}</ref> |
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{{clade |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1={{clade |
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|label1=Empire Prokaryota |
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|1={{clade |
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|1=Kingdom Monera |
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}} |
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}} |
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|2={{clade |
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|label1=Empire Eukaryota |
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|1={{clade |
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|1=Kingdom Protista |
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|2=Kingdom Plantae |
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|3=Kingdom Animalia |
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}} |
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}} |
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}} |
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}} |
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}} |
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==Five kingdoms== <!-- This section is linked from [[Monera]] --> |
==Five kingdoms== <!-- This section is linked from [[Monera]] --> |
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[[Robert Whittaker]] recognized an additional kingdom for the [[Fungus|Fungi]]. The resulting |
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.<ref name=Scamardella1999/> [[Robert Whittaker]] recognized an additional kingdom for the [[Fungus|Fungi]]. The resulting five-kingdom system, proposed in 1969, has become a popular standard and with some refinement is still used in many works and forms the basis for newer multi-kingdom systems. It is based mainly on differences in [[nutrition]]; his Plantae were mostly multicellular [[autotroph]]s, his Animalia multicellular [[heterotroph]]s, and his Fungi multicellular [[saprotroph]]s. The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies.<ref name="Whittaker1969">{{Citation |last=Whittaker |first=R.H. |year=1969 |title=New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms |journal=Science |volume=163 |issue=863 |pages=150–60 |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=5762760 |month=January |pmid=5762760 }}</ref> The five kingdom system may be combined with the two empire system. |
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{{clade |
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==Six kingdoms== |
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|1={{clade |
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<!-- This section is linked from [[Monera]] --> |
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|label1=life |
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In the years around 1980, there was an emphasis on phylogeny and redefining the kingdoms to be monophyletic groups, groups made up of relatively closely related organisms. The Animalia, Plantae, and Fungi were generally reduced to core groups of closely related forms, and the others placed into the Protista. Based on [[RNA]] studies, [[Carl Woese]] divided the prokaryotes (Kingdom [[Monera]]) into two kingdoms, called [[bacterium|Eubacteria]] and [[Archaea|Archaebacteria]]. Carl Woese attempted to establish a "Three Primary Kingdom" (or "urkingdom") system in which Animalia, Plantae, Protista, and Fungi were lumped into one primary kingdom of all eukaryotes. The Eubacteria and Archaebacteria made up the other two urkingdoms. The initial use of "six-kingdom systems" 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.<ref name="Woese1977">{{cite journal |author=Balch WE, Magrum LJ, Fox GE, Wolfe RS, Woese CR |title=An ancient divergence among the bacteria |journal=J. Mol. Evol. |volume=9 |issue=4 |pages=305–11 |year=1977 |month=August |pmid=408502 |doi=10.1007/BF01796092 |url=}}</ref> |
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|1={{clade |
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|1={{clade |
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|label1=Empire Prokaryota |
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|1={{clade |
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|1=Kingdom Monera |
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}} |
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}} |
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|2={{clade |
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|label1=Empire Eukaryota |
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|1={{clade |
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|1=Kingdom Protista |
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|2=Kingdom Plantae |
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|3=Kingdom Fungi |
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|4=Kingdom Animalia |
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}} |
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}} |
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}} |
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}} |
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}} |
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==Six kingdoms==<!-- This section is linked from [[Monera]] --> |
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A variety of new eukaryotic kingdoms were also proposed, but most were quickly invalidated, ranked down to phyla or classes, or abandoned. The only one which is still in common use is the kingdom [[Chromista]] proposed by [[Thomas Cavalier-Smith|Cavalier-Smith]], including organisms such as [[kelp]], [[diatom]]s, and [[water mould]]s. Thus the eukaryotes are divided into three primarily heterotrophic groups, the Animalia, Fungi, and Protozoa, and two primarily photosynthetic groups, the Plantae (including [[red alga|red]] and [[green alga]]e) and Chromista. However, it has not become widely used because of uncertainty over the monophyly of the latter two kingdoms. |
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From around the mid-1970s onwards, there was an increasing emphasis on molecular level comparisons of genes (initially [[ribosome|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 (Kingdom Monera) into two groups, called [[bacterium|Eubacteria]] and [[Archaea|Archaebacteria]], stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes. Eukaryote groups, such as plants, fungi and animals may look different, but are more similar to each other in their genetic makeup at the molecular level 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.) Woese attempted to establish a "three primary kingdom" or "urkingdom" system.<ref name="Woese1977">{{Citation |last=Balch |first=W.E. |last2=Magrum |first2=L.J. |last3=Fox |first3=G.E. |last4=Wolfe |first4=C.R. |year=1977 |title=An ancient divergence among the bacteria |journal=J. Mol. Evol. |volume=9 |issue=4 |pages=305–11 |doi=10.1007/BF01796092 |coauthors=& Woese, C.R. |month=August |pmid=408502 }}</ref> In 1990, the name "domain" was proposed for the highest rank.<ref>Woese C, 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. doi:10.1073/pnas.87.12.4576. PMID 2112744. PMC 54159.</ref> The initial use of a six-kingdom system 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. |
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{{clade |
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==Recent proposals== |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1={{clade |
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|label1=Domain Bacteria |
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|1={{clade |
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|1=Kingdom Bacteria |
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}} |
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}} |
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|2={{clade |
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|label1=Domain Archaea |
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|1={{clade |
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|1=Kingdom Archaea |
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}} |
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}} |
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|3={{clade |
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|label1=Domain Eukarya |
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|1={{clade |
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|1=Kingdom Protista |
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|2=Kingdom Plantae |
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|3=Kingdom Fungi |
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|4=Kingdom Animalia |
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}} |
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}} |
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}} |
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}} |
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}} |
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Woese also recognized that the Protista kingdom was not a monophyletic group and might be further divided at the level of kingdom. |
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Kingdom classification is in flux due to ongoing research and discussion. As new findings and technologies become available they allow the refinement of the model. For example, gene sequencing techniques allow the comparison of the genome of different groups ([[Phylogenomics]]). |
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==Traditional kingdoms disappear== |
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[[Image:Eukaryota tree.svg|thumb|350px|One hypothesis of eukaryotic relationships]] |
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The six-kingdom system is still recognizably an expansion 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. |
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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 argued that only monophyletic groups – an ancestor and ''all'' of its descendents – should be accepted as formal ranks in a classification. On this basis, the diagram opposite (redrawn from their article) showed the real 'kingdoms' (their quotation marks) of the eukaryotes.<ref>The real ‘kingdoms’ of eukaryotes Current Biology, Volume 14, Issue 17, 7 September 2004, Pages R693-R696 Alastair G.B. Simpson, Andrew J. Roger</ref> A classification produced in 2005 for the International Society of Protistologists, which reflected the consensus of the time, followed this approach, dividing the eukaryotes into the same six 'supergroups'.<ref name=Adl2005> |
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{{cite |
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| first = Sina M. |
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| last = Adl |
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| coauthors = ''et al.'' |
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| title = The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists |
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| journal = Journal of Eukaryotic Microbiology |
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| year=2005 | volume=52 | issue=5 | pages=399 |
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| url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1550-7408.2005.00053.x |
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| doi = 10.1111/j.1550-7408.2005.00053.x |
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}} |
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</ref> Although the published classification deliberately did not use formal taxonomic ranks, other sources have treated each of the six as a separate Kingdom. |
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{{clade |
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|1={{clade |
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|label1=life |
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|1={{clade |
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|1={{clade |
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|label1=Domain Bacteria |
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|1={{clade |
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|1=Bacteria |
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}} |
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}} |
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|2={{clade |
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|label1=Domain Archaea |
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|1={{clade |
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|1=Archaea |
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}} |
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}} |
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|3={{clade |
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|label1=Domain Eukarya |
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|1={{clade |
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|1=[[Amoebozoa]] – most lobose [[amoeboid]]s and [[slime mould]]s |
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|2=[[Opisthokonta]] – [[animal]]s, [[fungus|fungi]], [[choanoflagellate]]s, etc. |
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|3=[[Rhizaria]] – [[Foraminifera]], [[Radiolaria]], and various other [[amoeboid]] protozoa |
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|4=[[Archaeplastida]] (or Primoplantae) – [[Embryophyte|Land plants]], [[green alga]]e, [[red alga]]e, and [[glaucophyte]]s |
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|5=[[Chromalveolate|Chromalveolata]] – [[Heterokont|Stramenopiles]] (or Heterokonta), [[haptophyte|Haptophyta]], [[cryptomonad|Cryptophyta]] (or cryptomonads), and [[alveolate|Alveolata]] |
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|6=[[Excavate|Excavata]] – Various [[flagellate]] protozoa |
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}} |
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}} |
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}} |
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}} |
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}} |
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In this system, the traditional kingdoms have vanished. For example, since research shows that the multicellular [[land plants]] are descended from a group within the [[green alga]]e, they cannot form a valid rank in a monophyletic classification system. The monophyletic group is the [[Archaeplastida]], made up of all those organisms believed to have descended from a common ancestor, some of which are unicellular, some of which are multicellular but not closely related to land plants (e.g. green seaweeds, such as the [[Ulvophyceae]]), and others of which are traditional multicellular plants.<ref name=Adl2005/> |
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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,<ref>Harper, J. T., Waanders, E. & Keeling, P. J. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. Int. J. System. Evol. Microbiol., 55, 487-496. [http://www.botany.ubc.ca/keeling/PDF/05chromalvJSEM.pdf]</ref> and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.<ref name="parfrey">{{cite journal | url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1713255 | title = Evaluating Support for the Current Classification of Eukaryotic Diversity | author = Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz | doi = 10.1371/journal.pgen.0020220 | journal = PLoS Genet. | date = 2006 December | volume = 2 | issue = 12 | pages = e220 | pmid = 17194223 }}</ref> |
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{{As of|2010|April}}, there appears to be a consensus that the six supergroup model does not reflect the true phylogeny of the eukaryotes, although there is no agreement as to the model which should replace it.<ref>{{cite journal|url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000790|title=Phylogenomics Reshuffles the Eukaryotic Supergroups|author=Fabien Burki, Kamran Shalchian-Tabrizi, Marianne Minge, Åsmund Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan Pawlowski|journal=PLoS ONE|volume=2|issue=8|doi=10.1371/journal.pone.0000790|year=2007|pages=e790}}</ref><ref name=Burki2008>{{Citation |last=Burki |first=Fabien |last2=Shalchian-Tabrizi |first2=Kamran |last3=Pawlowski |first3=Jan |year=2008 |title=Phylogenomics reveals a new 'megagroup' including most photosynthetic eukaryotes |journal=Biology Letters |volume=4 |pages=366–369 |doi=10.1098/rsbl.2008.0224 |lastauthoramp=yes }}</ref><ref name=KimGraham2008>{{cite journal |author=Kim E, Graham LE |title=EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata |journal=PLoS ONE |volume=3 |issue=7 |pages=e2621 |year=2008 |pmid=18612431 |pmc=2440802 |doi=10.1371/journal.pone.0002621 }}</ref> |
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==Summary== |
==Summary== |
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{{biological systems}} |
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The sequence from the two-kingdom system to the six-kingdom system can be summarized in the table below. |
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{{biological systems}} |
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Note that the equivalences in this table are not perfect. e.g. Haeckel placed the [[red algae]] (Haeckel's Florideae; modern [[Florideophyceae]]) and [[blue-green algae]] (Haeckel's Archephyta; modern [[Cyanobacteria]]) in his Plantae. |
Note that the equivalences in this table are not perfect. e.g. Haeckel placed the [[red algae]] (Haeckel's Florideae; modern [[Florideophyceae]]) and [[blue-green algae]] (Haeckel's Archephyta; modern [[Cyanobacteria]]) in his Plantae. |
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However, research in the 21st century does not support the classification of the eukaryotes in any of these systems. {{As of|2010|April}}, the situation appears to be that there is no set of kingdoms sufficiently supported by current research to gain widespread acceptance; as Roger & Simpson say: "with the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."<ref>{{cite journal |author=Roger AJ, Simpson AGB. |title=Evolution: Revisiting the Root of the Eukaryote Tree |journal=Current Biology |volume=19 |issue=4 |pages=R165–7 |year=2009 |pmid=19243692|doi=10.1016/j.cub.2008.12.032 }}</ref> |
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In 1998, [[Cavalier-Smith]]<ref name="Cavalier-Smith1998">{{cite journal |author=Cavalier-Smith T |title=A revised six-kingdom system of life |journal=Biological Reviews of the Cambridge Philosophical Society |volume=73 |issue=3 |pages=203–66 |year=1998 |month=August |pmid=9809012 |doi=10.1111/j.1469-185X.1998.tb00030.x |url=http://www3.interscience.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=1464-7931&date=1998&volume=73&issue=3&spage=203}}</ref> proposed that Protista should be divided into 2 new kingdoms: [[Chromista]] the phylogenetic group of golden-brown algae that includes those algae whose chloroplasts contain chlorophylls a and c, as well as various colorless forms that are closely related to them, and [[Protozoa]], the kingdom of protozoans<ref name="Cavalier-Smith2006">{{cite journal | author = Cavalier-Smith T | title = Protozoa: the most abundant predators on earth | journal = Microbiology Today | url=http://www.sgm.ac.uk/pubs/micro_today/pdf/110605.pdf | format=pdf | volume = | pages = 166–7 | year = 2006 | doi = }}</ref>. This proposal has not been widely adopted, although the question of the relationships between different domains of life remains controversial.<ref>{{cite journal |author=Walsh DA, Doolittle WF |title=The real 'domains' of life |journal=Current Biology |volume=15 |issue=7 |pages=R237–40 |year=2005 |month=April |pmid=15823519 |doi=10.1016/j.cub.2005.03.034 |url=http://linkinghub.elsevier.com/retrieve/pii/S0960-9822(05)00300-3}}</ref> |
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{| |
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|----- |
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! Empires |
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! Kingdom (biology)|Kingdoms |
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|----- |
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| bgcolor="lightgrey" rowspan="1" | '''[[Prokaryota]]''' |
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| bgcolor="lavender" | '''[[Bacteria]]''' |
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|----- |
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| bgcolor="e0d0b0" rowspan="4" | '''[[Eukaryota]]''' |
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| bgcolor="pink" | '''[[Animalia]]''' |
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| bgcolor="lightgreen" | '''[[Plantae]]''' |
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| bgcolor="lightblue" | '''[[Fungi]]''' |
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| bgcolor="BurlyWood" | '''[[Chromista]]''' |
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| bgcolor="khaki" | '''[[Protozoa]]''' |
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|} |
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==See also== |
==See also== |
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Revision as of 16:07, 28 April 2010
In biology, kingdom or regnum 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. The complete sequence of ranks is life, domain, kingdom, phylum, class, order, family, genus, and species.
Currently, many textbooks from the United States use a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea, Bacteria) while British and Australian textbooks may describe five kingdoms (Animalia, Plantae, Fungi, Protista, and Prokaryota or Monera).
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.
Two kingdoms
The classification of living things into animals and plants is an ancient one. Thus Aristotle (384 BC – 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 (the History of Plants).[1]
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.
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Three kingdoms
In 1674, Antonie van Leeuwenhoek, often called the "father of microscopy", had sent the Royal Society of London a copy of his first observations of microscopic single-celled organisms. Up to this time, 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-1800s it had become clear that "the existing dichotomy of the plant and animal kingdoms [had] became rapidly blurred at its boundaries and outmoded".[2]. 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).[2]
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Four kingdoms
The development of microscopy, and the electron microscope in particular revealed an important distinction between those unicellular organisms whose cells did not have a distinct nucleus, prokaryotes, and those unicellular and multicellular organisms whose cells did 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.[2]
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It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized Édouard Chatton's proposal in the 1960s to recognize this division in a formal classification. This required the creation, for the first time, of a rank above kingdom, superkingdom or empire.[3]
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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.[2] Robert Whittaker recognized an additional kingdom for the Fungi. The resulting five-kingdom system, proposed in 1969, has become a popular standard and with some refinement is still used in many works and forms the basis for newer 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.[4] The five kingdom system may be combined with the two empire system.
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Six kingdoms
From around the mid-1970s onwards, there was an increasing emphasis on molecular level comparisons of genes (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 (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. Eukaryote groups, such as plants, fungi and animals may look different, but are more similar to each other in their genetic makeup at the molecular level 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.) Woese attempted to establish a "three primary kingdom" or "urkingdom" system.[5] In 1990, the name "domain" was proposed for the highest rank.[6] The initial use of a six-kingdom system 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.
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Woese also recognized that the Protista kingdom was not a monophyletic group and might be further divided at the level of kingdom.
Traditional kingdoms disappear
The six-kingdom system is still recognizably an expansion 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 argued that only monophyletic groups – an ancestor and all of its descendents – should be accepted as formal ranks in a classification. On this basis, the diagram opposite (redrawn from their article) showed the real 'kingdoms' (their quotation marks) of the eukaryotes.[7] A classification produced in 2005 for the International Society of Protistologists, which reflected the consensus of the time, followed this approach, dividing the eukaryotes into the same six 'supergroups'.[8] Although the published classification deliberately did not use formal taxonomic ranks, other sources have treated each of the six as a separate Kingdom.
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In this system, the traditional kingdoms have vanished. For example, since research shows that the multicellular land plants are descended from a group within the green algae, they cannot form a valid rank in a monophyletic classification system. The monophyletic group is the Archaeplastida, made up of all those organisms believed to have descended from a common ancestor, some of which are unicellular, some of which are multicellular but not closely related to land plants (e.g. green seaweeds, such as the Ulvophyceae), and others of which are traditional multicellular plants.[8]
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,[9] and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.[10]
As of April 2010[update], there appears to be a consensus that the six supergroup model does not reflect the true phylogeny of the eukaryotes, although there is no agreement as to the model which should replace it.[11][12][13]
Summary
The sequence from the two-kingdom system to the six-kingdom system can be summarized in the table below.
| Linnaeus 1735[14] |
Haeckel 1866[15] |
Chatton 1925[16] |
Copeland 1938[17] |
Whittaker 1969[4] |
Woese et al. 1990[18] |
Cavalier-Smith 1998,[19] 2015[20] |
|---|---|---|---|---|---|---|
| 2 kingdoms | 3 kingdoms | 2 empires | 4 kingdoms | 5 kingdoms | 3 domains | 2 empires, 6/7 kingdoms |
| (not treated) | Protista | Prokaryota | Monera | Monera | Bacteria | Bacteria |
| Archaea | Archaea (2015) | |||||
| Eukaryota | Protoctista | Protista | Eucarya | "Protozoa" | ||
| "Chromista" | ||||||
| Vegetabilia | Plantae | Plantae | Plantae | Plantae | ||
| Fungi | Fungi | |||||
| Animalia | Animalia | Animalia | Animalia | Animalia |
Note that the equivalences in this table are not perfect. e.g. Haeckel placed the red algae (Haeckel's Florideae; modern Florideophyceae) and blue-green algae (Haeckel's Archephyta; modern Cyanobacteria) in his Plantae.
However, research in the 21st century does not support the classification of the eukaryotes in any of these systems. As of April 2010[update], the situation appears to be that there is no set of kingdoms sufficiently supported by current research to gain widespread acceptance; as Roger & Simpson say: "with the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."[21]
See also
References
- ^ Singer, Charles J. (1931), A short history of biology, a general introduction to the study of living things, Oxford: Clarendon Press, OCLC 1197036
- ^ a b c d Scamardella, Joseph M. (1999), "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista", International Microbiology, 2: 207–16
- ^ Stanier, R.Y.; Van Neil, C.B. (1962), "The concept of a bacterium", Archiv Für Mikrobiologie, 42: 17–35, PMID 13916221
{{citation}}: Unknown parameter|lastauthoramp=ignored (|name-list-style=suggested) (help) - ^ a b 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 (863): 150–60, PMID 5762760
{{citation}}: Unknown parameter|month=ignored (help) Cite error: The named reference "Whittaker1969" was defined multiple times with different content (see the help page). - ^ Balch, W.E.; Magrum, L.J.; Fox, G.E.; Wolfe, C.R. (1977), "An ancient divergence among the bacteria", J. Mol. Evol., 9 (4): 305–11, doi:10.1007/BF01796092, PMID 408502
{{citation}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ Woese C, 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. doi:10.1073/pnas.87.12.4576. PMID 2112744. PMC 54159.
- ^ The real ‘kingdoms’ of eukaryotes Current Biology, Volume 14, Issue 17, 7 September 2004, Pages R693-R696 Alastair G.B. Simpson, Andrew J. Roger
- ^ a b
Adl, Sina M. (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
{{citation}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Harper, J. T., Waanders, E. & Keeling, P. J. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. Int. J. System. Evol. Microbiol., 55, 487-496. [1]
- ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMID 17194223.
{{cite journal}}: Check date values in:|date=(help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link) - ^ Fabien Burki, Kamran Shalchian-Tabrizi, Marianne Minge, Åsmund Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan Pawlowski (2007). "Phylogenomics Reshuffles the Eukaryotic Supergroups". PLoS ONE. 2 (8): e790. doi:10.1371/journal.pone.0000790.
{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link) - ^ Burki, Fabien; Shalchian-Tabrizi, Kamran; Pawlowski, Jan (2008), "Phylogenomics reveals a new 'megagroup' including most photosynthetic eukaryotes", Biology Letters, 4: 366–369, doi:10.1098/rsbl.2008.0224
{{citation}}: Unknown parameter|lastauthoramp=ignored (|name-list-style=suggested) (help) - ^ Kim E, Graham LE (2008). "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata". PLoS ONE. 3 (7): e2621. doi:10.1371/journal.pone.0002621. PMC 2440802. PMID 18612431.
{{cite journal}}: CS1 maint: unflagged free DOI (link) - ^ Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
- ^ 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.
- ^ Copeland, H. (1938). "The kingdoms of organisms". Quarterly Review of Biology. 13 (4): 383–420. doi:10.1086/394568. S2CID 84634277.
- ^ 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. (1998). "A revised six-kingdom system of life". Biological Reviews. 73 (3): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012. S2CID 6557779.
- ^ Ruggiero, Michael A.; Gordon, Dennis P.; Orrell, Thomas M.; Bailly, Nicolas; Bourgoin, Thierry; Brusca, Richard C.; Cavalier-Smith, Thomas; Guiry, Michael D.; Kirk, Paul M.; Thuesen, Erik V. (2015). "A higher level classification of all living organisms". PLOS ONE. 10 (4): e0119248. Bibcode:2015PLoSO..1019248R. doi:10.1371/journal.pone.0119248. PMC 4418965. PMID 25923521.
- ^ Roger AJ, Simpson AGB. (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.