In biology, a kingdom or regnum is the top-level, or nearly the top-level, taxon of organisms in scientific classification. (Sometimes domain or empire has been used as the topmost level.) The kingdoms are then divided into smaller groups called phyla (for animals) or divisions (for plants).
In his Systema Naturae, first published in 1735, 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.
Two empires, 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 Chatton to propose a division of life into two empires: organisms with a nucleus in Eukaryota and organisms without in Prokaryota.
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 . Copeland's four-kingdom system placed all eukaryotes other than animals and plants in the kingdom Protista .
It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized Chatton's two-empire system in the 1960s .
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, 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 .
In the years around 1980 there was an emphasis on phylogeny and redefining the kingdoms to be monophyletic. The Animalia, Plantae, and Fungi were generally reduced to core groups of closely related forms, and the others thrown into the Protista. Based on rRNA studies Carl Woese divided the prokaryotes into two kingdoms, called Eubacteria and Archaebacteria. Such six-kingdom systems have become standard in many works .
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 algae) and Chromista. However, it has not become widely used because of uncertainty over the monophyly of the latter two kingdoms.
|Woese et al.
|Woese et al.|
(Note that the equivalences in this table are not perfect. For example, Haeckel placed the red algae (Haeckel's Florideae; modern Florideophyceae) and blue-green algae (Haeckel'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.)
- 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.
- E. Chatton (1937). Titres et travaux scientifiques. Sette, Sottano, Italy.
- H. F. Copeland (1938). "The kingdoms of organisms". Quart. Rev. Biol. 13: 383–420.
- 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.
- R. H. Whittaker (1969). "New concepts of kingdoms of organisms". Science 163: 150–160.
- 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