However, some multicellular organisms, such as seaweeds, were included in Protista because of their relationships to specific unicellular protists.
The five-kingdom system prevailed in biology for more than 20 years.
During the past three decades, systematists applied cladistic analysis to taxonomy, constructing cladograms based on molecular data.
These data led to the three-domain system of Bacteria, Archaea, and Eukarya as “superkingdoms.”
Bacteria differ from Archaea in many key structural, biochemical, and physiological characteristics.
Many microbiologists have divided the two prokaryotic domains into multiple kingdoms based on cladistic analysis of molecular data.
A second challenge to the five-kingdom system comes from systematists who are sorting out the phylogeny of the former members of the kingdom Protista.
Molecular systematics and cladistics have shown that the Protista is not monophyletic.
Some of these organisms have been split among five or more new kingdoms.
Others have been assigned to the Plantae, Fungi, or Animalia.
Clearly, taxonomy at the highest level is a work in progress.
There will be much more research before there is anything close to a new consensus for how the three domains of life are related and how many kingdoms should be included in each domain.
New data, including the discovery of new groups, will lead to further taxonomic remodeling.
Keep in mind that phylogenetic trees and taxonomic groupings are hypotheses that fit the best available data.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-18