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The kingdom-level classification of life is still widely employed as a useful way of grouping organisms, notwithstanding some problems with this approach: Kingdoms such as Protozoa represent grades rather than clades, and so are rejected by phylogenetic classification systems.
The initial targets of Cavalier-Smith's classification, the protozoa were classified as members of the animal kingdom, [12] and many algae were regarded as part of the plant kingdom. With growing awareness that the animals and plants embraced unrelated taxa, the use of the two kingdom system was rejected by specialists.
[1] [b] In the 21st century, the classification shifted toward a two-kingdom system of protists: Chromista (containing the chromalveolate, rhizarian and hacrobian groups) and Protozoa (containing excavates and all protists more closely related to animals and fungi). [2] The following groups contain protists.
While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: Euglenozoa, Amoebozoa, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa.
This classification remained widespread in the 19th and early 20th century, [7] and even became elevated to a variety of higher ranks, including phylum, subkingdom, kingdom, and then sometimes included within the similarly paraphyletic Protoctista or Protista.
[7] [9] In traditional classification schemes, Amoebozoa is usually ranked as a phylum within either the kingdom Protista [10] or the kingdom Protozoa. [11] [12] In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. [7]
Another source lists the taxonomy as Kingdom Protozoa, Phylum Dinoflagellata, Class Dinophyceae, Order Gonyaulacales, and Family Ceratiaceae. [4] The taxonomic information listed on the right includes Kingdom Chromalveolate. Thus, sources disagree on the higher levels of classification, but agree on lower levels.
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.