Kingdom (biology)
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:
\nThe 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
Cavalier-Smith, including groups such as the
brown algae,
diatoms, and
water molds. In their absence the Protista may be renamed Protozoa. \nHe also proposed a kingdom Archezoa for primitive eukaryotes that lack
mitochondria, but in later revisions abandoned both it and the Archaebacteria.
See also:
Binomial nomenclature,
Scientific classification,
Taxonomy
Footnote
\n¹ 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.
References
\n*Whittaker, R.H. (1959). On the broad classification of organisms. Quart. Rev. Biol. 34, 210-226.\n*Whittaker, R.H. (1969). New concepts of kingdoms of organisms. Science 163, 150-160.
Category:Scientific classification\n\n\n\n\n\n\n