10.46 The Domain in Biology - Advanced (CK-12)

What do you wan…  Subjects Explore  Donate Sign In Sign Up 10.46 The Domain in Biology - Advanced FlexBooks 2.0 > CK-12 Advanced Biology > The Domain in Biology - Advanced Written by: Douglas Wilkin, Ph.D. | Niamh Gray-Wilson Fact-checked...

What do you wan…  Subjects Explore  Donate Sign In Sign Up 10.46 The Domain in Biology - Advanced FlexBooks 2.0 > CK-12 Advanced Biology > The Domain in Biology - Advanced Written by: Douglas Wilkin, Ph.D. | Niamh Gray-Wilson Fact-checked by: The CK-12 Editorial Team  Last Modified: Oct 02, 2024  [Figure 1] What's a domain?  Start Practice.com,.org,.edu are domain names. They can immediately classify a site into a general category. So can the domains Bacteria, Archaea, and Eukayra. The Domain Carl Woese wasn’t completely happy with the six-kingdom system. It didn’t show that all four eukaryote kingdoms are more closely related to each other than to the two bacteria kingdoms. It also didn’t show that the two bacteria kingdoms are as different from each other as they are from the eukaryote kingdoms. To show these similarities and differences, Woese introduced a new taxon called the domain. He defined domain as a taxon higher than the kingdom. The Three-Domain System In 1990, Woese and his colleagues proposed a new classification system containing three domains: Bacteria, Archaea, and Eukarya. As shown in Figure below, the Bacteria domain was formerly the  Eubacteria kingdom, and the Archaea domain was formerly the Archaebacteria kingdom. The  Eukarya domain includes all four eukaryote kingdoms: plants, animals, protists, and fungi. The three-domain system emphasizes the similarities among eukaryotes and the differences among eukaryotes, bacteria, and archaea. By using domains, Woese was able to show these relationships without replacing the popular six-kingdom system.  [Figure 2] This diagram shows how the three-domain system of classification is related to the six-kingdom system. Both the Eubacteria and Archaebacteria kingdoms are raised to the level of domains (Bacteria and Archaea domains, respectively) in the three-domain system. The other four kingdoms make up the third domain (Eukarya domain). Archaea were first found in extreme environments. For example, they were found in the hot water geysers in Yellowstone National park. Archaea have since been found in all of Earth’s habitats. They are now known to be present everywhere and in high numbers. They may contribute as much as 20 percent to Earth’s total biomass. Woese’s three-domain system was quickly adopted by many other biologists. There were some critics, however, who argued that the system put too much emphasis on the uniqueness of Archaea. Later studies confirmed how different Archaea are from other organisms. For example, organisms belogning to Archaea were found to differ from both Eukarya and Bacteria in the composition of  their cell membranes and the system they use for DNA replication. These differences convinced  most critics that the three-domain system was justified. After its introduction in 1990, the three- domain system became increasingly popular. Within a decade of its introduction, it had largely replaced earlier classifications. How Are the Three Domains Related? Comparing ribosomal RNA base sequences, Woese and his colleagues also showed that organisms belonging to Eukarya are more similar to Archaea than they are to Bacteria. Figure below is a phylogenetic tree based on their analysis. This tree places Archaea and Eukarya in the same clade. It represents the hypothesis that Archaea and Eukarya shared a more recent common ancestor with each other than either shared with Bacteria.  [Figure 3] This phylogenetic tree is based on comparisons of ribosomal RNA base sequences among living organisms. The   tree divides all organisms into three domains: Bacteria, Archaea, and Eukarya. Humans and other animals belong to the Eukarya domain. From this tree, organisms that make up the domain Eukarya appear to have shared a more recent common ancestor with Archaea than Bacteria. The results of a study published in 2007 seem to conflict with this hypothesis. Comparing DNA base sequences, the 2007 study suggested that the domain Archaea may be older than either Bacteria or Eukarya. That would make Archaea the most ancient group of organisms on Earth. It is not yet known, which, if either, hypothesis is correct. Scientists need to learn more about Archaea and their relationships with other organisms to resolve these questions. The Future of Classification