In biological taxonomy, a kingdom or regnum
is a taxon in either (historically) the highest rank, or (in the new three-domain system) the rank below domain. Each kingdom is divided into smaller groups called phyla (or in
some contexts these are called "divisions"). Currently, textbooks from the United States use a system of six kingdoms
(Animalia, Plantae, Fungi, Protoctista, Archaea, and
Monera), while British and Australian textbooks describe five kingdoms (Animalia, Plantae, Fungi,
Protista, Bacteria).
Carolus Linnaeus distinguished two kingdoms of living things: Animalia for
animals and Vegetabilia for plants (Linnaeus
also treated minerals, placing them in a third kingdom, Mineralia). Linnaeus divided each
kingdom into classes, later grouped into phyla for animals and divisions for plants. When single-celled organisms were first discovered, they were split between the
two kingdoms: mobile forms in the animal phylum Protozoa, and colored algae and bacteria in the plant division Thallophyta or Protophyta. However, a
number of forms were hard to place, or were placed in different kingdoms by different authors: for example, the mobile alga
Euglena and the amoeba-like slime moulds. As a result, Ernst Haeckel suggested creating a third
kingdom Protista for them.[1][2]
Two superkingdoms, Four kingdoms
The discovery that bacteria have a radically different cell structure from other organisms —
the bacterial cell has one or two membranes that lie at or near its surface, whereas other organisms have a more complex
structure with a nucleus and other organelles divided by
intracellular membranes — led microbiologist Edouard Chatton to propose a division of life into organisms with a nucleus in
Eukaryota and organisms without in Prokaryota.[3]
Chatton's proposal was not taken up immediately; a more typical system was that of Herbert
Copeland, who gave the prokaryotes a separate kingdom, originally called Mychota but later referred to as Monera or Bacteria.[4] Copeland's four-kingdom system placed all eukaryotes other than animals and plants in the
kingdom Protista.[5]
It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized
Chatton's proposal in the 1960s.[6]
Five kingdoms
Robert Whittaker recognized an additional kingdom for the Fungi. The resulting five-kingdom system, proposed in 1968, has become a popular standard and with some
refinement is still used in many works, or 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.[7]
Six kingdoms
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 rRNA studies Carl Woese divided the
prokaryotes (Kingdom Monera) into two kingdoms, called Eubacteria and Archaebacteria. Carl Woese attempted to establish a Three Primary Kingdom (or Urkingdom) system in which Plants,
Animals, 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 Kingdom system. Such six Kingdom systems have become standard in many works.[8]
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
Cavalier-Smith, including organisms such as kelp,
diatoms, and water moulds. Thus the eukaryotes are divided into
three primarily heterotrophic groups, the Animalia, Fungi, and Protozoa, and two primarily photosynthetic groups, the Plantae
(including red and green algae) and Chromista. However,
it has not become widely used because of uncertainty over the monophyly of the latter two kingdoms.
Woese stresses genetic similarity over outward appearances and behaviour, relying on comparisons of ribosomal RNA genes at the
molecular level to sort out classification categories. A plant does not look like an animal, but at the cellular level, both
groups are eukaryotes, having similar subcellular organization, including cell nuclei, which the Eubacteria and Archaebacteria do
not have. More importantly, plants, animals, fungi, and protists are more similar to each other in their genetic makeup at the
molecular level, based on rRNA studies, than they are to either the Eubacteria or Archaebacteria. Woese also found that all of
the eukaryotes, lumped together as one group, are more closely related, genetically, to the Archaebacteria than they are to the
Eubacteria. This means that the Eubacteria and Archaebacteria are separate groups even when compared to the eukaryotes. So, Woese
established the Three-domain system, clarifying that all the Eukaryotes are more
closely genetically related compared to their genetic relationship to either the bacteria or the archaebacteria, without having
to replace the "six kingdom systems" with a three kingdom system. The Three Domain system is a "six kingdom system" that unites
the eukaryotic kingdoms into the Eukarya Domain based on their relative genetic similarity when compared to the Bacteria Domain
and the Archaea Domain. Woese also recognized that the Protista Kingdom is not a monophyletic group and might be further divided
at the level of Kingdom. Others have divided the Protista Kingdom into the Protozoa and the Chromista, for instance.
Summary
(Note that the equivalences in this table are not perfect. e.g. Haeckell placed the red algae
(Haeckell's Florideae; modern Florideophyceae) and blue-green algae (Haeckell's Archephyta; modern Cyanobacteria) in
his Plantae, but in modern classifications they are considered protists and bacteria respectively. However, despite this and
other failures of equivalence, the table gives a useful simplification; empires are erroneously attributed to Chatton in the
table who did not rank the 2 groups nor formally name them).
References
- ^ a b E. Haeckel (1866). Generelle
Morphologie der Organismen. Reimer, Berlin.
- ^ Joseph M. Scamardella
(1999). "Not plants
or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista". International Microbiology
2: 207–221.
- ^ a b E. Chatton (1937). Titres et
travaux scientifiques. Sette, Sottano, Italy.
- ^ H. F. Copeland
(1938). "The kingdoms of organisms". Quart. Rev. Biol. 13: 383–420.
- ^ a b H. F. Copeland (1956).
The Classification of Lower Organisms. Palo Alto: Pacific Books.
- ^ R. Y. Stanier and C. B. van
Niel (1962). "The concept of a bacterium". Arch. Microbiol. 42: 17–35.
- ^ a b R. H. Whittaker (1969). "New concepts
of kingdoms of organisms". Science 163: 150–160.
- ^ a b C. R. Woese, W. E. Balch, L. J. Magrum, G.
E. Fox and R. S. Wolfe (1977). "An ancient divergence among the bacteria". Journal of Molecular Evolution 9:
305–311.
- ^ Carl R. Woese, Otto Kandler, Mark L. Wheelis: "Towards a
Natural System of Organisms: Proposal for the domains Archaea, Bacteria, and Eucarya", doi:10.1073/pnas.87.12.4576
See also
External links
- The five kingdom concept
- Whittaker's classification
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