The 6 Kingdoms of Biology

The three domains of life[edit]

A phylogenetic tree based on rRNA data showing Woese's three-domain system. All smaller branches can be considered kingdoms.

From around the mid-1970s onwards, there was an increasing emphasis on comparisons of genes on the molecular level (initially ribosomal RNA genes) as the primary factor in classification; genetic similarity was stressed over outward appearances and behavior. Taxonomic ranks, including kingdoms, were to be groups of organisms with a common ancestor, whether monophyletic (all descendants of a common ancestor) or paraphyletic (only some descendants of a common ancestor). Based on such RNA studies, Carl Woese, thought life could be divided into three large divisions and referred to them as the "three primary kingdom" model or "urkingdom" model.[4] In 1990, the name "domain" was proposed for the highest rank.[5] Woese divided the prokaryotes (previously classified as the Kingdom Monera) into two groups, called Eubacteria and Archaebacteria or Archaea, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes.

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. Although the primacy of the Eubacteria-Archaea divide has been questioned, it has been upheld by subsequent research.[6] There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.

How many kingdoms are present in the Eukarya?[edit]

One hypothesis of eukaryotic relationships, modified from Simpson and Roger (2004).

In 2004, a review article by Simpson and Roger noted that the Protista were "a grab-bag for all eukaryotes that are not animals, plants or fungi". They held that only monophyletic groups should be accepted as formal ranks in a classification and that, while this approach had been impractical previously (necessitating "literally dozens of eukaryotic ‘kingdoms’"), it had now become possible to divide the eukaryotes into "just a few major groups that are probably all monophyletic". On this basis, the diagram opposite (redrawn from their article) showed the real 'kingdoms' (their quotation marks) of the eukaryotes.[7] A classification which followed this approach was produced in 2005 for the International Society of Protistologists, by a committee which "worked in collaboration with specialists from many societies". It divided the eukaryotes into the same six "supergroups".[8] The published classification deliberately did not use formal taxonomic ranks, including that of "kingdom".