Explain the concept of a synapomorphy and outline how shared derived characters are used to group organisms into clades.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A synapomorphy is a shared derived character that is inherited from a common ancestor and is present in all members of a clade but not in organisms outside that clade.
Shared derived characters are distinguished from ancestral (plesiomorphic) characters, which are inherited from a more distant ancestor and shared more broadly – only derived characters support grouping into a clade.
Organisms that share one or more synapomorphies are grouped together into a clade because the shared derived character is evidence of descent from a common ancestor in which the character first evolved.
Multiple synapomorphies can be used to define nested clades, so that smaller clades (sharing more recent derived characters) are contained within larger clades (sharing older derived characters), reflecting evolutionary history.
Shared derived characters are distinguished from ancestral (plesiomorphic) characters, which are inherited from a more distant ancestor and shared more broadly – only derived characters support grouping into a clade.
Organisms that share one or more synapomorphies are grouped together into a clade because the shared derived character is evidence of descent from a common ancestor in which the character first evolved.
Multiple synapomorphies can be used to define nested clades, so that smaller clades (sharing more recent derived characters) are contained within larger clades (sharing older derived characters), reflecting evolutionary history.
Examiner tips
- Use the term "synapomorphy" first, then explain it as a shared derived character. Show the contrast with plesiomorphy. Explain that shared derived characters define clades. Mention nested clades to cover all four points.
Common mistakes
- Confusing synapomorphy with any shared trait, including plesiomorphy. Failing to state that the character must be present in all members of the clade. Omitting the idea of nested clades or the evolutionary significance of derived characters.
Mark scheme (4 marks)
- A synapomorphy is a shared derived character (trait) that is inherited from a common ancestor and is present in all members of a clade but not in organisms outside that clade.
- Shared derived characters are distinguished from ancestral (plesiomorphic) characters, which are inherited from a more distant ancestor and shared more broadly — only derived characters support grouping into a clade.
- Organisms that share one or more synapomorphies are grouped together into a clade because the shared derived character is evidence of descent from a common ancestor in which the character first evolved.
- Multiple synapomorphies can be used to define nested clades, so that smaller clades (sharing more recent derived characters) are contained within larger clades (sharing older derived characters), reflecting evolutionary history.
Key terms in this question
synapomorphy · clade · derived character
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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