Classification of living organisms — AQA GCSE Biology
The Linnaean system, binomial names, the three-domain system and evolutionary trees.
Revision notes
2 short notes, in the order of the specification. Each one in short:
The traditional classification system groups living things by their structure and characteristics into kingdom, phylum, class, order, family, genus and species. Organisms are named by the binomial system of genus and species, for example Homo sapiens. The genus is written first with a capital letter, the species second with a lower-case letter, and the name is italicised or underlined.
The three-domain system divides organisms into Archaea, Bacteria and Eukaryota, based on evidence from chemical analysis. Archaea are primitive bacteria usually living in extreme environments, Bacteria are true bacteria, and Eukaryota include protists, fungi, plants and animals. New models of classification were proposed as better microscopes revealed internal structures and the understanding of biochemical processes progressed.
Exam questions
3 exam-style questions (6 marks), each with its mark scheme.
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The whole of inheritance, variation and evolution on one page, so you can see where this subtopic fits.
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Quick check questions
List the Linnaean groups from largest to smallest.
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Kingdom, phylum, class, order, family, genus, species.
Which two groups make up a binomial name?
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Genus and species.
Name the three domains in Woese's system.
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Archaea, bacteria and eukaryota.
What data are used to build evolutionary trees?
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Current classification data for living organisms and fossil data for extinct organisms.
Frequently asked questions
What is the binomial system of naming organisms?
The binomial system names each organism by two parts: its genus and its species. For humans, the genus is Homo and the species is sapiens, giving Homo sapiens. The genus is written first with a capital letter and the species second with a lower-case letter, and the whole name is written in italics or underlined.
What are the seven levels of classification?
The seven levels of classification, from largest to smallest, are kingdom, phylum, class, order, family, genus and species. Living things were traditionally classified into these groups depending on their structure and characteristics. The last two groups, genus and species, are used to give each organism its two-part binomial name, such as Homo sapiens.
What is the three-domain system?
The three-domain system is a model of classification that divides organisms into Archaea, Bacteria and Eukaryota. It was proposed because of evidence from chemical analysis. Archaea are primitive bacteria usually living in extreme environments, Bacteria are true bacteria, and Eukaryota include protists, fungi, plants and animals.
How do you read an evolutionary tree?
To read an evolutionary tree, find the point where two branches split from a common ancestor. The nearer that point is to the present, the more closely related the two organisms are. Evolutionary trees show how scientists believe organisms are related, using current classification data for living organisms and fossil data for extinct organisms.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026