View source for Kingdom (biology)

{{pp-vandalism|small=yes}}
{{Biological classification}}
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 (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]].

Currently, textbooks from the United States use a system of six kingdoms ([[Animal]]ia, [[Plantae]], [[Fungi]], [[Protist]]a, [[Archaea]] and [[Bacteria]]) while British, Australian and Latin American 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. [[Aristotle]] (384–322 BC) classified animal species in his work [[History of Animals|''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)).<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>

[[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 [[animal]]s 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.

{{clade
|1={{clade
|label1=life
|1={{clade
|1=Regnum Vegetabile
|2=Regnum Animale
}}
}}
}}

==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".<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 |pmid=10943416 |issue=4 }}</ref> In 1866, following earlier proposals by [[Richard Owen]] and [[John Hogg (biologist)|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/>

{{clade
|1={{clade
|label1=life
|1={{clade
|1=Kingdom Protista
|2=Kingdom Plantae
|3=Kingdom Animalia
}}
}}
}}

==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 [[Cell nucleus|nucleus]], [[prokaryote]]s, and those unicellular and multicellular organisms whose cells do 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/>

{{clade
|1={{clade
|label1=life
|1={{clade
|1=Kingdom Monera (prokaryotes, i.e. bacteria and "blue-green algae")
|2=Kingdom Protista (single-celled eukaryotes)
|3=Kingdom Plantae
|4=Kingdom Animalia
}}
}}
}}

It gradually became apparent how important the prokaryote/eukaryote distinction is, and 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 (biology)|domain]]''.<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 |issue=1 |pages=17–35 |pmid=13916221 |lastauthoramp=yes |doi=10.1007/BF00425185 }}</ref>

{{clade
|label1=&nbsp;&nbsp;life&nbsp;
|1={{clade
   |label1=Empire&nbsp;Prokaryota
   |1={{clade
      |1=Kingdom Monera
      }}
   |label2=Empire&nbsp;Eukaryota
   |2={{clade
      |1=Kingdom Protista
      |2=Kingdom Plantae
      |3=Kingdom Animalia
      }}
   }}
}}

==Five kingdoms== <!-- This section is linked from [[Monera]] -->

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 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 [[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=3863 |pages=150–60 |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=5762760 |month=January |pmid=5762760 |doi=10.1126/science.163.3863.150|bibcode = 1969Sci...163..150W }}</ref> The five kingdom system may be combined with the two empire system.

{{clade
|1={{clade
|label1=life
|1={{clade
|1={{clade
|label1=Empire&nbsp;Prokaryota
|1={{clade
|1=Kingdom Monera
}}
}}
|2={{clade
|label1=Empire&nbsp;Eukaryota
|1={{clade
|1=Kingdom Protista
|2=Kingdom Plantae
|3=Kingdom Fungi
|4=Kingdom Animalia
}}
}}
}}
}}
}}

==Six kingdoms==<!-- This section is linked from [[Monera]] -->

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 [[bacteria|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.) Although the primacy of the eubacteria-archaebacteria divide has been questioned, it has also been upheld by subsequent research.<ref name="DagenEtAl2010">{{Citation |last=Dagan |first=T. |last2=Roettger |first2=M. |last3=Bryant |first3= |last4=Martin |first4=W. |year=2010 |title=Genome Networks Root the Tree of Life between Prokaryotic Domains |journal=Genome Biology and Evolution |volume=2: |issue=0 |pages=379–92 |doi=10.1093/gbe/evq025 |lastauthoramp=yes }}</ref>

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>{{Citation |last=Woese |first=C.R. |last2=Kandler |first2=O. |last3=Wheelis |first3=M. |year=1990 |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proc Natl Acad Sci U S A |volume=87 |issue=12 |pages=4576–9 |doi=10.1073/pnas.87.12.4576 |pmid=2112744 |pmc=54159 |lastauthoramp=yes |bibcode=1990PNAS...87.4576W}}</ref> 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.

{{clade
|1={{clade
|label1=life
|1={{clade
|1={{clade
|label1=Domain&nbsp;Bacteria
|1={{clade
|1=Kingdom Bacteria
}}
}}
|2={{clade
|label1=Domain&nbsp;Archaea
|1={{clade
|1=Kingdom Archaea
}}
}}
|3={{clade
|label1=Domain&nbsp;Eukarya
|1={{clade
|1=Kingdom Protoctista or Protista
|2=Kingdom Plantae
|3=Kingdom Fungi
|4=Kingdom Animalia
}}
}}
}}
}}
}}

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 six kingdoms===
{{See also|Thomas Cavalier-Smith}}
[[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.<ref>{{Citation |last=Palaeos.com |title=Origins of the Eukarya |url=http://www.palaeos.com/Eukarya/Eukarya.html |accessdate=2010-04-29 |archiveurl=http://www.webcitation.org/5pLfYf6U7 |archivedate=2010-04-29 }}</ref> In 1998, he published a six-kingdom model,<ref>{{Citation |last=Cavalier-Smith |first=T. |year=1998 |title=A revised six-kingdom system of life |journal=Biological Reviews |volume=73 |pages=203–66 |url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=685 |issue=03 |doi=10.1111/j.1469-185X.1998.tb00030.x |pmid=9809012}}</ref> which has been revised in subsequent papers. The version published in 2009 is shown below.<ref name="CavalierSmith2009">{{Citation |last=Cavalier-Smith |first=Thomas |author-link=Thomas Cavalier-Smith |year=2009 |title=Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree |journal=Biology Letters |volume=6 |issue=3 |pages=342&ndash;5 |doi=10.1098/rsbl.2009.0948 |pmc=2880060 |pmid=20031978}}</ref> (Compared to the version he published in 2004,<ref name="CavalierSmith2004">{{Citation |last=Cavalier-Smith |first=T. |year=2004 |title=Only six kingdoms of life |journal=Proc. R. Soc. Lond. B |volume=271 |pages=1251–62 |url=http://www.cladocera.de/protozoa/cavalier-smith_2004_prs.pdf |accessdate=2010-04-29 |doi=10.1098/rspb.2004.2705 |pmid=15306349 |issue=1545 |pmc=1691724 }}</ref> the [[Alveolata|alveolates]] and the [[rhizaria]]ns have been moved from Kingdom Protozoa to Kingdom Chromista.) Cavalier-Smith does not accept the importance of the fundamental eubacteria–archaebacteria divide put forward by Woese and others and supported by recent research.<ref name="DagenEtAl2010"/> His Kingdom Bacteria includes the Archaebacteria as part of a subkingdom along with a group of eubacteria (Posibacteria). Nor does he accept the requirement for groups to be [[Monophyly|monophyletic]]. His Kingdom Protozoa includes the ancestors of Animalia and Fungi. Thus the diagram below does not represent an evolutionary tree.

{{clade
|1={{clade
|label1=life
|1={{clade
|1={{clade
|label1=Empire&nbsp;Prokaryota
|1={{clade
|1=Kingdom Bacteria &mdash; includes Archaebacteria as part of a subkingdom
}}
}}
|2={{clade
|label1=Empire&nbsp;Eukaryota
|1={{clade
|1=Kingdom Protozoa &mdash; e.g. [[Amoebozoa]], [[Choanozoa]], [[Excavata]]
|2=Kingdom Chromista &mdash; e.g. [[Alveolata]], [[cryptophyte]]s, [[Heterokonta]] (stramenopiles), [[Haptophyta]], [[Rhizaria]]
|3=Kingdom Plantae &mdash; e.g. [[glaucophyte]]s, [[red alga|red]] and [[green alga]]e, [[land plants]]
|4=Kingdom Fungi
|5=Kingdom Animalia
}}
}}
}}
}}
}}

==International Society of Protistologists Classification 2005==
[[Image:Eukaryota tree.svg|thumb|350px|One hypothesis of eukaryotic relationships, modified from Simpson and Roger (2004).]]

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 argued that only monophyletic groups–an ancestor and ''all'' of its descendents &mdash; 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>{{Citation |last=Simpson |first=Alastair G.B. |last2=Roger |first2=Andrew J. |year=2004 |title=The real ‘kingdoms’ of eukaryotes |journal=Current Biology |volume=14 |issue=17 |pages=R693–6 |lastauthoramp=yes |pmid=15341755 |doi=10.1016/j.cub.2004.08.038 }}</ref> 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".<ref name=Adl2005>
 {{Citation
 | first = Sina M.
 | last = Adl
 | coauthors = ''et al.''
 | title = The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists
 | journal = Journal of Eukaryotic Microbiology
 | year=2005 | volume=52 | issue=5 | pages=399
 | url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1550-7408.2005.00053.x
 | doi = 10.1111/j.1550-7408.2005.00053.x
 | pmid=16248873
}}
</ref> Although the published classification deliberately did not use formal taxonomic ranks, other sources{{citation needed|date=December 2010}} have treated each of the six as a separate kingdom.

{{clade
|1={{clade
|label1=life
|1={{clade
|1={{clade
|label1=Domain&nbsp;Bacteria
|1={{clade
|1=Bacteria
}}
}}
|2={{clade
|label1=Domain&nbsp;Archaea
|1={{clade
|1=Archaea
}}
}}
|3={{clade
|label1=Domain&nbsp;Eukarya
|1={{clade
|1=[[Excavate|Excavata]] &mdash; Various [[flagellate]] protozoa
|2=[[Amoebozoa]] &mdash; most lobose [[amoeboid]]s and [[slime mould]]s
|3=[[Opisthokonta]] &mdash; [[animal]]s, [[fungus|fungi]], [[choanoflagellate]]s, etc.
|4=[[Rhizaria]] &mdash; [[Foraminifera]], [[Radiolaria]], and various other [[amoeboid]] protozoa
|5=[[Chromalveolate|Chromalveolata]] &mdash; [[Heterokont|Stramenopiles]] (or Heterokonta), [[haptophyte|Haptophyta]], [[cryptomonad|Cryptophyta]] (or cryptomonads), and [[alveolate|Alveolata]]
|6=[[Archaeplastida]] (or Primoplantae) &mdash; [[Embryophyte|Land plants]], [[green alga]]e, [[red alga]]e, and [[glaucophyte]]s
}}
}}
}}
}}
}}

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 [[choanoflagellate]]s 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.<ref name=Adl2005/>

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>{{Citation |last=Harper |first=J.T. |last2=Waanders |first2=E. |last3=Keeling |first3=P. J. |year=2005 |title=On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes |journal=Nt. J. System. Evol. Microbiol. |volume=55 |issue=Pt 1 |pmid=15653923 |pages=487–496 |url=http://www.botany.ubc.ca/keeling/PDF/05chromalvJSEM.pdf |lastauthoramp=yes |doi=10.1099/ijs.0.63216-0 }}</ref> and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.<ref name="parfrey">{{Citation |last=Parfrey |first=Laura W. |last2=Barbero |first2=Erika |last3=Lasser |first3=Elyse |last4=Dunthorn |first4=Micah |year=2006 |coauthors=Bhattacharya, Debashish; Patterson, David J. & Katz, Laura A. |title=Evaluating Support for the Current Classification of Eukaryotic Diversity |journal=PLoS Genet. |volume=2 |issue=12 |pages=e220 |doi=10.1371/journal.pgen.0020220 |pmid=17194223 |pmc=1713255 }}</ref>

{{As of|2010}}, there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a clade dubbed the [[SAR supergroup]],<ref name="Burki2007p4">{{Harvnb|Burki|Shalchian-Tabrizi|Minge|Skjæveland|2007|p=4}}</ref> so that Rhizara is not one of the main eukaryote groups.<ref name="CavalierSmith2009"/><ref name="Burki2007"/><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|pmid=18522922 |volume=4 |issue=4 |pmc=2610160 |pages=366–369 |doi=10.1098/rsbl.2008.0224|lastauthoramp=yes |postscript=.}}</ref><ref name="Burki2009">{{Citation |last=Burki |first=F. ''et al.'' |last2=Inagaki |year=2009 |first2=Y. |last3=Brate |first3=J. |last4=Archibald |first4=J. M. |last5=Keeling |first5=P. J. |last6=Cavalier-Smith |first6=T. |last7=Sakaguchi |first7=M. |last8=Hashimoto |first8=T. |last9=Horak |first9=A. |title=Large-Scale Phylogenomic Analyses Reveal That Two Enigmatic Protist Lineages, Telonemia and Centroheliozoa, Are Related to Photosynthetic Chromalveolates |journal=Genome Biology and Evolution |volume=1 |issue=0 |pages=231&ndash;8 |doi=10.1093/gbe/evp022 |pmc=2817417 |pmid=20333193}}</ref><ref name="Hackett2007">{{Citation |last=Hackett|first=J.D. |last2=Yoon |first2=H.S. |last3=Li |first3=S. |last4=Reyes-Prieto |first4=A.|coauthors=Rummele, S.E. & Bhattacharya, D.|year=2007 |title=Phylogenomic analysis supports the monophyly of cryptophytes and haptophytes and the association of Rhizaria with chromalveolates |journal=Mol. Biol. Evol. |volume=24 |issue=8 |pages=1702&ndash;13|doi=10.1093/molbev/msm089 |pmid=17488740}}</ref> 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."<ref>{{Citation |last=Rogozin |first=I.B. |last2=Basu |first2=M.K. |last3=Csürös |first3=M.|last4=Koonin |first4=E.V. |year=2009 |title=Analysis of Rare Genomic Changes Does Not Support the Unikont–Bikont Phylogeny and Suggests Cyanobacterial Symbiosis as the Point of Primary Radiation of Eukaryotes |journal=Genome Biology and Evolution |volume=1 |issue=0 |pages=99–113 |doi=10.1093/gbe/evp011 |pmc=2817406 |lastauthoramp=yes |pmid=20333181}}</ref> {{As of|2010|December}}, 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.<ref name="Burki2007">{{Citation |last=Burki |first=Fabien |last2=Shalchian-Tabrizi |first2=Kamran |last3=Minge |first3=Marianne |last4=Skjæveland |first4=Åsmund |coauthors=Nikolaev, Sergey I.; Jakobsen, Kjetill S. & Pawlowski, Jan |editor1-last=Butler |year=2007 |editor1-first=Geraldine |title=Phylogenomics Reshuffles the Eukaryotic Supergroups |journal=PLoS ONE |volume=2 |issue=8 |pages=e790 |url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000790 |doi=10.1371/journal.pone.0000790 |pmid=17726520 |pmc=1949142 |bibcode = 2007PLoSO...2..790B }}</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 |pmid=18522922 |issue=4 |pmc=2610160 }}</ref><ref name=KimGraham2008>{{Citation |last=Kim |first=E. |last2=Graham |first2=L.E. |year=2008 |title=EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata |journal=PLoS ONE |volume=3 |editor1-first=Rosemary Jeanne |issue=7 |pages=e2621 |editor1-last=Redfield |doi=10.1371/journal.pone.0002621 |pmid=18612431 |pmc=2440802 |lastauthoramp=yes |last3=Redfield |first3=Rosemary Jeanne |bibcode = 2008PLoSO...3.2621K }}</ref>

==Summary==
The sequence from the two-kingdom system up to Cavalier-Smith's six-kingdom system can be summarized in the table below.
{{biological systems}}
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.

One or other of the kingdom-level classifications of life is still widely employed as a useful way of grouping organisms, notwithstanding the problems with this approach:
* Kingdoms such as Bacteria represent [[evolutionary grade|grade]]s rather than [[clade]]s, and so are rejected by [[phylogenetic classification]] systems.
* Research in the 21st century does not support the classification of the eukaryotes into ''any'' of the standard 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>{{Citation |last=Roger |first=A.J. |last2=Simpson |first2=A.G.B. |year=2009 |title=Evolution: Revisiting the Root of the Eukaryote Tree |journal=Current Biology |volume=19 |issue=4 |pages=R165–7 |doi=10.1016/j.cub.2008.12.032 |pmid=19243692 |lastauthoramp=yes }}</ref>

==See also==
* [[Cladistics]]
* [[Systematics]]

==References==
{{reflist|2}}

==External links==
* [http://waynesword.palomar.edu/trfeb98.htm The five kingdom concept]
* [http://home.manhattan.edu/~frances.cardillo/plants/intro/plntlist.html Whittaker's classification]

{{Taxonomic ranks}}

{{DEFAULTSORT:Rank01}}
[[Category:Scientific classification]]
[[Category:Botanical nomenclature|Kingdom]]
[[Category:Plant taxonomy|1rank01]]
[[Category:Zoological nomenclature]]
[[Category:Biology terminology|Kingdom]]