How Are Living Organisms Classified?
Learn how living organisms are classified for AQA GCSE Biology, from the Linnaean groups and binomial names to the three-domain system and evolutionary trees.
Living organisms are classified by grouping them according to shared characteristics and evidence of how they are related. For AQA GCSE Biology, the key systems are Linnaean classification and the newer three-domain system. This applies to the separate Biology GCSE at both Foundation and Higher tier.
The Linnaean system
The Linnaean system places organisms into groups that become more specific in this order: kingdom, phylum, class, order, family, genus, species. Organisms in the same species are more closely grouped than organisms that share only a kingdom. Each step narrows the group: a genus contains organisms grouped more closely together than a family does, while a species is the most specific group in the sequence.
Each organism has a binomial name made from its genus and species. For example, humans are named Homo sapiens: Homo is the genus and sapiens identifies the species. Using these two names gives scientists a consistent way to identify an organism.
Why classification has changed
Scientists originally classified living things mainly by their visible structures and characteristics. Improvements in microscopes revealed more about organisms’ internal structures. Chemical analysis also provided new evidence, leading Carl Woese to develop the three-domain system. A domain is a broad group used in this newer system. Its three domains are archaea, bacteria and eukaryota. Eukaryota includes protists, fungi, plants and animals.
Scientists can also use an evolutionary tree to show how they believe organisms are related. These trees draw on classification data from living organisms and fossil evidence from extinct organisms. When reading a tree, follow its branches to see which organisms are shown as sharing a more recent common ancestor. The tree represents scientists’ interpretation of the available evidence.
Exam mistake to avoid: do not list the three domains as if they are steps in the Linnaean sequence. The Linnaean sequence runs from kingdom to species; the three-domain system is a different way of grouping organisms based on later evidence.
