In biological taxonomy, a domain ( or ) (Latin: regio), also dominion, or two domains, Archaea and Bacteria, with Eukarya included in Archaea. In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus. All organisms that have a cell nucleus and other membrane-bound organelles are included in Eukarya and called eukaryotes.

Non-cellular life, most notably the viruses, is not included in this system. Alternatives to the three-domain system include the earlier two-empire system (with the empires Prokaryota and Eukaryota), and the eocyte hypothesis (with two domains of Bacteria and Archaea, with Eukarya included as a branch of Archaea).

Terminology

The term domain was proposed by Carl Woese, Otto Kandler, and Mark Wheelis (1990) in a three-domain system.

Development of the domain system

Carl Linnaeus made the classification "domain" popular in the famous taxonomy system he created in the middle of the eighteenth century. This system was further improved by the studies of Charles Darwin later on but could not classify bacteria easily, as they have very few observable features to compare to the other domains.

The mycologist Royall T. Moore proposed the similar term dominion () in 1974, but this was not widely adopted. His proposed system had three dominions: Virus, Prokaryota, and Eukaryota.

Carl Woese made a revolutionary breakthrough when, in 1977, he compared the nucleotide sequences of the 16s ribosomal RNA and discovered that the rank "domain" contained three branches, not two as scientists had previously thought. Initially, due to their physical similarities, Archaea (earlier "archaebacteria") was not separated from Bacteria (earlier "eubacteria"). However, scientists now know that these two domains are hardly similar and are internally distinctly different.

Characteristics of the three domains

200px|thumb|The three-domain tree and the [[eocyte hypothesis (two-domain tree), 2008.]]

thumb|200px|[[Phylogenetic tree showing the relationship between the eukaryotes and other forms of life, 2006. Eukaryotes are colored red, archaea green, and bacteria blue.]]

Each of these three domains contains unique ribosomal RNA. This forms the basis of the three-domain system. While the presence of a nuclear membrane differentiates the Eukarya from the Archaea and Bacteria, both of which lack a nuclear envelope, the Archaea and Bacteria are distinct from each other due to differences in the biochemistry of their cell membranes and RNA markers.

Alternative classifications

Alternative classifications of life include:

  • The two-empire system, proposed by Mayr (1998), with top-level groupings of Prokaryota (or Monera) and Eukaryota.
  • The eocyte hypothesis, proposed by Lake et al. (1984),
  • The five-dominium system, proposed by Stefan Luketa (2012), The listed Eukarya, Archaea, Bacteria, Virusobiota, Prionobiota.