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

        Bacteria Domai

        • Eubacteria Kingdo

        Eukarya Domai

      • 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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