Explain why a taxon that is defined only by shared primitive characters (symplesiomorphies) is considered invalid in cladistic classification.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A taxon defined only by symplesiomorphies is invalid because symplesiomorphies are ancestral traits inherited from a distant common ancestor and are therefore shared by many unrelated groups. A valid clade must be defined by shared derived characters (synapomorphies) that arose in the most recent common ancestor of the group. If a group is based solely on symplesiomorphies it will either be paraphyletic—excluding some descendants that also possess the trait—or polyphyletic—grouping unrelated taxa that share the ancestral trait. Cladistics aims to represent true evolutionary relationships, so a grouping lacking shared derived characters does not reflect a real evolutionary lineage.
Examiner tips
- Use the terms "symplesiomorphy", "synapomorphy", "monophyletic", "paraphyletic", "polyphyletic".
- Show the logical link: ancestral trait → shared by many → not unique to group → invalid clade.
- Explain the consequence: exclusion or inclusion of unrelated taxa.
- Keep answer to 4 clear points, each matching a mark point.
Common mistakes
- Confusing symplesiomorphy with synapomorphy; using the wrong term.
- Failing to mention that the group would be paraphyletic or polyphyletic.
- Not explaining why ancestral traits are shared by many groups.
Mark scheme (4 marks)
- Symplesiomorphies are ancestral characters inherited from a distant common ancestor and therefore shared by many groups beyond the taxon in question.
- A valid clade (monophyletic group) must be defined by shared derived characters (synapomorphies) that originated in the most recent common ancestor of the group.
- A group defined only by symplesiomorphies would be paraphyletic or polyphyletic, excluding descendants that also inherited those characters or including distantly related taxa.
- Cladistics requires groups to reflect true evolutionary (phylogenetic) relationships / common descent, so a grouping not supported by shared derived characters does not represent a real evolutionary lineage.
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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