Domain (taxonomy)
Highest taxonomic rank of all organisms.
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In biological taxonomy, a domain (sometimes called a dominion, from Latin *dominium* meaning 'lordship' or 'property') is the broadest classification rank for all living organisms. This rank was introduced in 1990 by Carl Woese, Otto Kandler, and Mark Wheelis as part of their three-domain system. Depending on the model used, the tree of life is divided into either three domains—Archaea, Bacteria, and Eukarya—or, under the two-domain system (often conflated with the eocyte hypothesis), Archaea and Bacteria, with Eukarya nested within Archaea. Non-cellular life forms, especially viruses, are not covered by this system.
- alternatives
- Two-empire system, eocyte hypothesis
Lore & Background
The term domain was proposed by Carl Woese, Otto Kandler, and Mark Wheelis in 1990 in a three-domain system. Despite earlier taxonomic systems, Carl Linnaeus did not use the term 'domain' in his classification, and the difficulty with bacteria classification is unrelated to Charles Darwin's work. The mycologist Royall T. Moore proposed the similar term dominion in 1974, with three dominions: Virus, Prokaryota, and Eukaryota, but this was not widely adopted. A later fringe proposal by Stefan Luketa in 2012 suggested a five-dominion system adding Prionobiota and Virusobiota, but this has not gained acceptance in mainstream taxonomy.
Reader's Guide
The domain system represents a fundamental shift in biological classification, replacing the earlier two-empire system of Prokaryota and Eukaryota. Carl Woese's revolutionary breakthrough in 1977, comparing nucleotide sequences of 16s ribosomal RNA, revealed three branches instead of two. Initially, Archaea was not separated from Bacteria due to physical similarities, but scientists now know these two domains are distinctly different. Each domain contains unique ribosomal RNA, forming the basis of the three-domain system. The presence of a nuclear membrane differentiates Eukarya from Archaea and Bacteria, while Archaea and Bacteria differ in cell membrane biochemistry and RNA markers. Alternatives include the eocyte hypothesis, which places Eukarya within Archaea, and the five-dominion system proposed by Stefan Luketa in 2012, adding Prionobiota and Virusobiota.
Did You Know?
- The term domain was proposed by Carl Woese, Otto Kandler, and Mark Wheelis in 1990.
- Royall T. Moore proposed the similar term dominion in 1974, but it was not widely adopted.
- The three-domain system includes no form of non-cellular life, such as viruses.
- A fringe five-dominion system was proposed by Stefan Luketa in 2012, but it is not considered a mainstream alternative.
Frequently Asked Questions
What is a domain in biological taxonomy?
A domain is the topmost rank in the hierarchical classification of living organisms, sitting above kingdom and phylum. It was proposed in 1990 as a way to capture the deepest evolutionary splits among cellular life.
Who introduced the three-domain system?
The domain rank was proposed in 1990 by Carl Woese, Otto Kandler, and Mark Wheelis. Their framework reorganized the tree of life around fundamental differences in cellular and molecular biology.
What are the three domains of life?
In the classic three-domain model, all cellular organisms fall into Archaea, Bacteria, and Eukarya. Some alternative frameworks, like the eocyte hypothesis, collapse Eukarya into a subgroup of Archaea, leaving just two domains.
Do viruses belong to a domain?
No—viruses and other non-cellular entities are excluded from the domain system entirely. The three-domain scheme was designed specifically to classify organisms made of cells.
Is domain really the highest rank in taxonomy?
Yes, within the standard hierarchy, domain (sometimes called dominion) sits at the very top, above kingdom. A few alternative systems, such as the five-dominion or two-empire proposals, restructure things differently, but domain remains the most widely recognized top-level rank.
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