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 for animals, and the Vegetabilia or Plantae for plants. Early authors also treated minerals in a third kingdom Mineralia. Each kingdom was divided into classes, later grouped into phyla for animals and divisions for plants. More recently, various other kingdoms have been created, and the ideal number and delineations are not settled.
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 placed in both - for instance the mobile alga Euglena, and the amoeba-like slime moulds. As a result, Ernst Haeckel suggested creating a third kingdom Protista for them, although this was not very popular until relatively recently (sometimes as the Protoctista).
The discovery that bacteria have a radically different cell structure from other organisms (prokaryotic rather than eukaryotic) led Copeland to give them a separate kingdom, originally called Mychota but later referred to as Monera or Bacteria. As it became apparent how important this distinction is, it became popular to divide living things into two superkingdoms or empires, called Prokaryota and Eukaryota.
Whittacker recognized an additional kingdom for the Fungi. The resulting five-kingdom system, proposed in 1959, has became 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 kingdom 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, which comprises heterokonts such as kelp, diatoms, and water moulds together with a few close relatives. 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.
