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In Biology , a kingdom or '''regnum''' is the top-level, or nearly the top-level, Taxon of organisms in Scientific Classification . In his ''s and Vegetabilia for Plant s (Linnaeus also treated Mineral s, 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 Alga e 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 Mould s. As a result, Ernst Haeckel suggested creating a third kingdom Protist a for them, although this was not very popular until relatively recently (sometimes as the Protoctista ) 12. TWO EMPIRES, FOUR KINGDOMS The discovery that and organisms without in Prokaryota .3 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 4. Copeland's four-kingdom system placed all eukaryotes other than animals and plants in the kingdom Protoctista 5. 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 6. FIVE KINGDOMS s, his Animalia multicellular Heterotroph s, and his Fungi multicellular Saprotroph s. The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies 7. SIX KINGDOMS 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 8. 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 , Diatom s, and Water Mould s. Thus the eukaryotes are divided into three primarily heterotrophic groups, the Animalia, Fungi, and Protozoa, and two primarily photosynthetic groups, the Plantae (including Red Alga e) and Chromista. However, it has not become widely used because of uncertainty over the monophyly of the latter two kingdoms. THREE DOMAINS In 1990, Carl Woese proposed that the Eubacteria, Archaebacteria, and Eukaryota represent three primary lines of descent and accordingly he promoted them to domains, naming them Bacteria, '''Archaea''', and '''Eucarya''' 9. This ''' Three-domain System ''' has received notable criticism but has generally displaced the older two-empire system as a way of organizing kingdoms together 10. SUMMARY (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.) REFERENCES |
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