Kingdom

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Kingdom

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Rubus Berry Plants

Rubus Berry Plants

Kingdom

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 below domain. Kingdoms are divided into smaller groups called phyla (in zoology) or divisions in botany. The complete sequence of ranks is Life, Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Currently, textbooks from the United States use a system of six Kingdoms (Animalia, Plantae, Fungi, Protista, Archaea, 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 BC–322 BC) classified animal species in his work the History of Animals, and his pupil Theophrastus (c. 371–c. 287 BC) wrote a parallel work on plants (the History of Plants).

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 animals and Regnum Vegetabile ('vegetable Kingdom') for plants. (Linnaeus also included minerals, placing them in a third Kingdom, Regnum Lapideum.) Linnaeus divided each Kingdom into classes, later grouped into phyla for animals and divisions for plants.

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. Up to this time, 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 rapidly blurred at its boundaries and outmoded". In 1866, following earlier proposals by Richard Owen and 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).

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 nucleus, prokaryotes, and those unicellular and multicellular organisms whose cells do have a distinct nucleus, eukaryotes. In 1938, Herbert F. Copeland proposed a four-Kingdom classification, moving the two prokaryotic groups, bacteria and "blue-green algae", into a separate Kingdom Monera.

It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized Édouard Chatton's proposal in the 1960s 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.

Five kingdoms

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. Robert Whittaker recognized an additional Kingdom for the 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 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. The five Kingdom system may be combined with the two empire system.

Six kingdoms

From around the mid-1970s onwards, there was an increasing emphasis on molecular level comparisons of genes (initially 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 Eubacteria and 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.

Woese attempted to establish a "three primary Kingdom" or "urkingdom" system. In 1990, the name "domain" was proposed for the highest rank. 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.

Woese also recognized that the Protista Kingdom was not a monophyletic group and might be further divided at the level of Kingdom.

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