Three Domain System

• This system is an evolutionary model of classification, it uses the Linnaean system, but adds an extra taxon above Kingdom – domain
• Based on differences in the sequences of nucleotides in the cell's ribosomal RNAs (rRNA) and the cell's membrane lipid structure
• rRNA structure


  • Useful for comparing as rRNA molecules throughout nature carry out the same function and their structure changes very little over time
  • Similarities/dissimilarities in rRNA nucleotide sequence are a good indication of how related/unrelated different cells and organisms are
• Domain system


  Proposes that a common ancestor cell gave rise to three different cell types, each representing a domain
• The three domains


  • The Archaea (archaebacteria)
  • The Bacteria (eubacteria)
  • The Eukarya (eukaryotes)
• The Eukarya are divided into the 4 kingdoms


  • Protoctists
  • Fungi
  • Animalia
  • Plantae
• Phylogeny


  The evolutionary history of organisms
• Another way of looking at classification is by observing the phylogeny or the evolutionary history of organisms