How are living organisms classified, and how has classification changed over time?
The Linnaean system of classification, the binomial naming system, how evidence from DNA has changed classification, the three-domain system, and the use of evolutionary trees.
A focused answer to AQA GCSE Biology 4.6.4, covering the Linnaean classification system, the binomial naming system, how DNA evidence changed classification, the three-domain system, and evolutionary trees.
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What this dot point is asking
AQA wants you to describe the Linnaean system of classification and binomial naming, explain how new evidence (especially DNA) has changed classification, describe the three-domain system, and interpret evolutionary trees.
The Linnaean system
Each group divides into smaller, more specific groups, ending with the species, which is the smallest group. Organisms in the same species are alike enough to breed together and produce fertile offspring. A useful way to remember the order is a mnemonic where the first letters spell out a phrase you can recall in the exam.
The binomial naming system
How classification has changed
As microscopes improved, biologists could see the internal structures of cells in detail, and as biochemistry developed they understood the biochemical processes within cells, especially the sequence of bases in DNA. This new evidence showed how organisms are really related through evolution, so classification systems were changed to reflect these evolutionary relationships rather than just outward appearance.
Evolutionary trees
Evolutionary trees are a method scientists use to show how they believe organisms are related. They use current classification data for living organisms and fossil data for extinct organisms. Each branch point represents a common ancestor; organisms that share a more recent common ancestor are more closely related. Because the trees combine evidence from living organisms (their characteristics and DNA) with fossil evidence for extinct organisms, they can be revised as new evidence is found, which is why classification is treated as a developing model rather than a fixed list.
The move from Linnaeus to Woese shows how science changes with new evidence. Linnaeus could only use features he could see, so organisms that looked similar were grouped together. As tools improved, biologists could compare the internal structure of cells and, crucially, the sequence of bases in DNA. DNA comparison is powerful because the more similar the base sequences of two organisms, the more closely related they are, regardless of outward appearance. This revealed that the simple-looking Archaea are genetically very different from ordinary Bacteria, which is why Woese separated them into different domains. AQA wants you to be able to explain that classification reflects evolutionary relationships and changes as the evidence improves.
Try this
Q1. Write the seven groups of the Linnaean system in order from largest to smallest. [2 marks]
- Cue. Kingdom, phylum, class, order, family, genus, species.
Q2. Name the three domains in Woese's system. [3 marks]
- Cue. Archaea, Bacteria and Eukaryota.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksExplain why the system used to classify living organisms has changed since Linnaeus first developed his system.Show worked answer →
A 3-mark explain question rewards the new evidence and its effect.
When Linnaeus developed his system, organisms were classified mainly on their observable structure. Since then, microscopes have improved, so the internal structures of cells could be seen, and the biochemistry of cells, especially the sequence of bases in DNA, became understood. This new evidence showed how organisms are really related, so the classification was changed to reflect their evolutionary relationships, for example Woese proposing the three-domain system.
Markers reward improved microscopes and understanding of cell chemistry or DNA as the new evidence, and the idea that classification was changed to show true relationships.
AQA 20214 marksAn evolutionary tree shows the relationships between five species. Describe what an evolutionary tree shows, and explain how you would use it to decide which two species are most closely related.Show worked answer →
A 4-mark question rewards interpreting the tree.
An evolutionary tree shows how scientists believe organisms are related, using classification data for living organisms and fossil data for extinct ones. Each branch point represents a common ancestor, and the tips represent species.
Two species are most closely related if they share the most recent common ancestor, which is the branch point closest to them; on the tree these two species join together lower down (later) than they join to the others. The shorter the path back to a shared branch point, the more closely related the species are.
Markers reward the tree showing relationships through common ancestors, and identifying the closest pair as those sharing the most recent common ancestor.
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Sources & how we know this
- AQA GCSE Biology (8461) specification — AQA (2016)