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In biology, a kingdom is the top-level, or nearly the top-level, grouping of organisms in scientific classification. Originally, two kingdoms were distinguished, the Animalia (animals), which typically could move about, and the Plantae (plants), which typically could not. Early authors also had a third kingdom for minerals. Each kingdom was divided into classes, later into phyla for animals and divisions for plants. This simplistic classification has been largely abandoned thanks to new developments, however.
When single-celled organisms were first discovered, they were split between the two kingdoms: motile forms were placed in the phylum Protozoa, while colored forms (algae) and bacteria were categorized in several divisions of plants. A number of forms ended up being placed in both - for instance Euglena and slime molds. As a result, a third kingdom, the Protista, was created to hold these groups. This was first suggested by Ernst Haeckel, though it was some time before the kingdom gained much currency.
Copeland introduced a fourth kingdom for bacteria, which have a prokaryotic cell organization rather than the eukaryotic organization found in his other three kingdoms. He called them the Mychota, but this was later replaced with Monera from their primitive form. The fungi, which he included among the Protoctista (an alternate name for the Protista), were given their own kingdom by Whittaker. Thus, he had three kingdoms for multicellular organisms, depending on whether they were autotrophic (Plantae), saprotrophic (Fungi), or heterotrophic (Animalia), and two for unicellular or colonial organisms (Protista and Monera). With some variation in the exact circumscription of these groups, this five-kingdom system has been standard for a long time, and is still used in many works.
However, newer findings have led to alternative systems. Most notable was the finding by Carl Woese that prokaryotes comprised two distinct groups, which he called Eubacteria and Archaebacteria but are now called Bacteria and Archaea, which are not or are not much closer related to each other than they are to the eukaryotes. This prompted the three-domain system, with each of these groups is treated as a domain. The domains were originally a replacement for kingdoms, but are more commonly used as a higher level rank, with the Eukaryota divided into several different kingdoms. Alternatively, some have simply treated the Bacteria and Archaea as two kingdoms in place of the Monera. This six-kingdom system has replaced the five-kingdom system in many works.
A comparison of the more notable systems:
The Protista have long been recognized as a junk-basket category for organisms that don't fit into the other eukaryotic kingdoms, and as a result some workers have promoted various protist groups to kingdoms. The most notable of these is the kingdom Chromista, proposed by Thomas Cavalier-Smith, which corresponds essentially to the heterokonts and includes groups like brown algae, diatoms, and water molds. He also proposed a kingdom Archezoa for primitive eukaryotes that lack mitochondria, but this gained little support due to the group's implausible heterogeneity, and in later revisions he abandoned both it and the Archaebacteria. Other groups which have been considered kingdoms, though considerably less often, include the alveolates and euglenozoa.
¹ R. H. Whittaker's Five Kingdoms system, first propounded in 1959, has remained substantially unchanged since then, but subsequent refinements were made (see References). The terms protoctista and prokaryotae are often substituted for protista and monera respectively.