Classification of Living Organisms — AQA GCSE Biology
Classification organises living things into groups based on their similarities. The system has developed as scientific knowledge has improved.
The Linnaean system
Traditionally, organisms were classified by Carl Linnaeus into groups based on their structure and characteristics. The hierarchy (largest to smallest group) is:
Kingdom → Phylum → Class → Order → Family → Genus → Species
A useful mnemonic is "King Philip Came Over For Good Soup".
Binomial naming
Species are named using the binomial system: the genus (capital letter) followed by the species (lower case), e.g. Homo sapiens. This gives every species a unique, internationally understood name.
Changing classification
As understanding of internal structures, biochemistry and especially DNA improved (with better microscopes and technology), classification systems have changed.
The three-domain system
Carl Woese proposed the three-domain system using evidence from DNA/RNA analysis. Organisms are divided into three domains:
- Archaea — primitive bacteria, often living in extreme environments.
- Bacteria — true bacteria.
- Eukaryota — protists, fungi, plants and animals.
Exam tips
- Learn the Linnaean hierarchy in order (Kingdom → … → Species).
- Use the binomial system correctly (Genus species).
- Explain that classification changed because of DNA evidence and improved technology.
- Know the three domains: Archaea, Bacteria, Eukaryota (proposed by Woese).