Phylogenetic Classification (AQA A Level Biology)

• Taxonomy is the practice of biological classification
• The phylogenetic classification system enables us to arrange species into groups based on their evolutionary origins and relationships
• There is no overlap between groups and each group is called a taxon (plural taxa)
• By grouping organisms into taxa it can make them easier to understand and remember
• There are several different ranks or levels within the hierarchical classification system used in biology
  • Multiple smaller taxa can be put in the same larger taxa

• The highest rank is the domain
• Cell type has a major role in the classification of organisms into the three domains; but do not confuse cell types and domain
  • Prokaryotic cells are easily distinguishable in that they lack a nucleus
  • Eukaryotic cells have compartmentalised structures, with at least their genetic material segregated from the rest of the cell in a nucleus

• Based upon molecular analysis of RNA genes in particular, scientists have realised that using cell type to classify organisms is insufficient, and that prokaryotes could be divided into two separate groups (domains)
• The three domains are:
  • Archaea (prokaryotes)
  • Bacteria (prokaryotes)
  • Eukarya (eukaryotes)

Archaea

• Organisms within this domain are sometimes referred to as the extremophile prokaryotes, archaea were first discovered living in extreme environments, but not all archaea do
• Archael cells have no nucleus (and so are prokaryotic)
• They were initially classified as bacteria until several unique properties were discovered that separated them from known bacteria, including:
  • Unique lipids being found in the membranes of their cells
  • No peptidoglycan in their cell walls
  • Ribosomal structure (particularly that of the small subunit) are more similar to the eukaryotic ribosome than that of the bacteria

• Archaea a similar size range as bacteria (and in many ways metabolism is similar between the two groups)
• DNA transcription is more similar to that of eukaryotes
• Example: Halobacterium salinarum are a species of the archaea domain that can be found in environments with high salt concentrations like the Dead Sea

Bacteria

• These are organisms that have prokaryotic cells which contain no nucleus
• They vary in size over a wide range: the smallest are bigger than the largest known-viruses and the largest are smaller that the smallest known single-celled eukaryotes
• Bacterial cells divide by binary fission
• Example: Staphylococcus pneumoniae is a bacteria species that causes pneumonia

Eukarya

• Organisms that have eukaryotic cells with nuclei and membrane-bound organelles are placed in this domain
• They vary massively in size from single-celled organisms several micrometres across to large multicellular organisms many-metres in size, such as blue whales
• Eukaryotic cells divide by mitosis
• Eukaryotes can reproduce sexually or asexually
• Example: Canis lupus also known as wolves

The three domains