From Regina Bailey,
Your Guide to Biology.
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The Three Domain System, developed by Carl Woese, is a system for classifying biological organisms.

Over the years, scientists have developed several systems for the classification of organisms. From the late 1960's, organisms had been classified according to a Five Kingdom system.

This classification system model was based on principles developed by Swedish scientist Carolus Linnaeus, whose hierarchical system groups organisms based on common physical characteristics.

The Three Domain System

As scientists learn more about organisms, classification systems change. Genetic sequencing has given researchers a whole new way of analyzing relationships between organisms.

The current system, the Three Domain System, groups organisms primarily based on differences in ribosomal RNA structure. Ribosomal RNA is a molecular building block for ribosomes.

Under this system, organisms are classified into three domains and six kingdoms. The domains are Archaea, Bacteria, and Eukarya. The kingdoms are Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia

The Archaea and Bacteria domains contain prokaryotic organisms. These are organisms that do not have a membrane bound nucleus.

Eubacteria are classified under the Bacteria domain and archaebacteria are classified as Archaeans.

The Eukarya domain includes eukaryotes, or organisms that have a membrane bound nucleus. This domain is further subdivided into the kingdoms Protista, Fungi, Plantae, and Animalia.

Comparison of Classification Systems

Five Kingdom System

Kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.

Three Domain System

Archaea Domain

• Archaebacteria Kingdom

Bacteria Domain

• Eubacteria Kingdom

Eukarya Domain

• Protista Kingdom
  
  
• Fungi Kingdom
  
  
• Plantae Kingdom
  
  
• Animalia Kingdom

As we have seen, systems for classifying organisms change with new discoveries made over time. The earliest systems recognized only two kingdoms (plant and animal).

The current Three Domain System is the best organizational system we have now, but as new information is gained, a different system for classifying organisms may later be developed.

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