Relate classification to common ancestry
Distinguish evolutionary relationships from superficial resemblance.
Section 1.2 · Concept and uses of classification systems
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does evolutionary classification aim to reflect?
What you will learn
- Explain the aim of evolutionary classification.
- Use several kinds of evidence to compare relatedness.
Groups can reflect shared descent
A classification system aims to reflect evolutionary relationships: organisms with a more recent common ancestor are more closely related. Structural features can provide evidence, but a similar habitat or body shape may develop in different lineages.
A bird and a bat both have wings, yet a bat has mammalian features. The flight similarity alone is insufficient to place the bat among birds. Compare multiple relevant features and genetic evidence rather than deciding from one conspicuous adaptation.
A tree is a hypothesis supported by evidence
An evolutionary diagram represents branching relationships, not a ladder of better organisms. Two organisms descending from the same recent branch point are more closely related than organisms whose shared branch point is earlier. Living organisms are not automatically direct ancestors of one another.
A key answers which supplied specimen this is; an evolutionary classification asks how groups are related. The order in which a key happens to ask questions does not supply dates or prove ancestry. New evidence may change a proposed relationship.
Pause and explain
What supports closer relatedness in a supplied tree?
Worked example
A simplified tree joins groups A and B at a later branch point than their shared branch with C. Which pair is more closely related?
Show the worked solution
- Locate the common ancestor of A and B.
- Compare it with the earlier ancestor shared with C.
- The more recent common ancestor supports closer relatedness.
Answer A and B in the supplied tree.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A tree joins X and Y before joining their branch with Z. Identify the closer pair.
Give me a hint
Compare the recency of shared branch points.
Compare my reasoning
- X and Y have the later common ancestor.
- The ancestor shared with Z is earlier.
- X and Y are closer in the supplied model.
X and Y.
Look for these in your work
- Use common-ancestor recency.
- Refer to the supplied tree.
A bat has wings, hair and milk-producing glands. Explain why flight does not make it a bird.
Give me a hint
Compare flight with mammalian diagnostic traits.
Compare my reasoning
- Wings are shared by different groups.
- Hair and milk-producing glands support mammals.
- One adaptation cannot override the fuller evidence.
The bat is a mammal; wing similarity alone does not establish bird membership.
Look for these in your work
- Give mammalian evidence.
- Avoid grouping by flight alone.
A key groups two specimens together because both are green. Why is their closest evolutionary relationship still unproven?
Give me a hint
Identification criteria may be superficial.
Compare my reasoning
- Green colour is a shared observation.
- It does not establish a recent common ancestor.
- Further structural and genetic evidence is needed.
The key shows an identification grouping, not proof of closest ancestry.
Look for these in your work
- Separate key use from evolutionary evidence.
- Name useful further evidence.
Common mistakes
- Wings or habitat alone can be misleading.
- Do not treat an identification key as a dated ancestry tree.
What can you explain now?
Does closer relatedness mean one living species must be the direct ancestor of the other?
Compare with the explanation
No.
They can share a more recent common ancestor without one being the direct ancestor of the other.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain closer relatedness using common ancestors, then contrast a tree with a key.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.