The three-domain system
(a) Less than ten years after the creation of the five-kingdom system of classification, microbiologist Carl Woese was instrumental in establishing a new system of classification which a little over ten years later became the three-domain system
(b) This system was basically accepted by microbiologists during the late 1980s, early 1990s and is increasing the system of choice of non-microbiologist biologists
(c) It even made the headlines a few years back with the declaration that a "new" form or life had been discovered (a.k.a., archaeobacteria, which had been discovered years previously and had been shown to be a "different" form of cellular life in the late 1970s, but one member of which was DNA sequenced in full in the late 1990s supplying the genesis of the headlines; with a complete sequence we obtained unambiguous confirmation of just how different from bacteria and eucaryotes these beasts truly are) [completely sequenced Archaeal genomes.
Domain
(a) The domain is a taxonomic category that, depending on point of view, is either above the level of kingdom (i.e., includes kingdoms within it) or supercedes the kingdom
(b) Regardless of viewpoint, the domain system contains three members
(i) Eukaryotes (domain Eukarya)
(ii) Eubacteria (domain Bacteria)
(iii) Archaebacteria (domain Archaea)
(c) A fourth domain or domain-like taxon, called the Urkaryotes, represents eukaryotes prior to their establishment of endosymbioses with eubacteria, i.e., mitochondria
KINGDOM
In biology, kingdom 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.
The five-kingdom system
(a) The five-kingdom system was first proposed in 1969 and is showing its age
(b) It posits the existence of five kingdoms (kingdom therefore being the highest/most inclusive taxonomic category in this system)
(c) The five kingdoms include:
(i) Plantae (the plants)
(ii) Fungi (the fungi)
(iii) Animalia (the animals)
(iv) Protista (the unicellular eucaryotes)
(v) Monera (the prokaryotes)
