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[3] [4] 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
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
According to genetic data, although eukaryote groups such as plants, fungi, and animals may look different, they are more closely related to each other than they are to either the Eubacteria or Archaea. It was also found that the eukaryotes are more closely related to the Archaea than they are to the Eubacteria.
Molecular phylogenetics has provided insight into the evolution and interrelationships of the three domains of life. The division between prokaryotes and eukaryotes reflects two very different levels of cellular organization; only eukaryotic cells have an enclosed nucleus that contains its DNA, and other membrane-bound organelles including ...
Cladistics is a method of classification of life forms according to the proportion of characteristics that they have in common (called synapomorphies). It is assumed that the higher the proportion of characteristics that two organisms share, the more recently they both came from a common ancestor.
Originally, Linnaeus established three kingdoms in his scheme, namely for Plants, Animals and an additional group for minerals, which has long since been abandoned. Since then, various life forms have been moved into three new kingdoms: Monera, for prokaryotes (i.e., bacteria); Protista, for protozoans and most algae; and Fungi.
Phylogenetic tree showing the relationship between the archaea and other forms of life. Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al. [44] Woese argued that the bacteria, archaea, and eukaryotes represent separate lines of descent that diverged early on from an ancestral colony of organisms.
Partial classifications exist for many individual groups of organisms and are revised and replaced as new information becomes available; however, comprehensive, published treatments of most or all life are rarer; recent examples are that of Adl et al., 2012 and 2019, [81] [82] which covers eukaryotes only with an emphasis on protists, and ...