Explain how the construction of a cladogram uses shared derived characters (synapomorphies) to determine evolutionary relationships between organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cladistics is a method of biological classification that groups organisms based on common ancestry, using observable characteristics to infer phylogenetic relationships.
Model answer (4 marks)
Synapomorphies are shared derived characters inherited from a common ancestor and are used to group organisms into clades.
Organisms that share more synapomorphies are considered more closely related and are placed closer together, or in the same clade, on the cladogram.
Each branching point (node) on a cladogram represents a hypothetical common ancestor at which a new derived character first appeared.
Cladograms are constructed using the principle of parsimony, selecting the tree that requires the fewest evolutionary changes to explain the distribution of characters observed.
Organisms that share more synapomorphies are considered more closely related and are placed closer together, or in the same clade, on the cladogram.
Each branching point (node) on a cladogram represents a hypothetical common ancestor at which a new derived character first appeared.
Cladograms are constructed using the principle of parsimony, selecting the tree that requires the fewest evolutionary changes to explain the distribution of characters observed.
Examiner tips
- Use the exact term "synapomorphies" and explain they are derived characters. Show the link between more shared synapomorphies and closer relationship. Mention nodes as common ancestors. State the principle of parsimony as the method for choosing the best tree.
Common mistakes
- Confusing synapomorphies with plesiomorphies or homoplasies. Failing to explain why more shared synapomorphies mean closer relatedness. Omitting the role of parsimony in tree construction.
Mark scheme (4 marks)
- Synapomorphies are shared derived characters that are inherited from a common ancestor and are used to group organisms into clades.
- Organisms that share more synapomorphies are considered more closely related and are placed closer together / in the same clade on the cladogram.
- Each branching point (node) on a cladogram represents a hypothetical common ancestor at which a new derived character first appeared.
- Cladograms are constructed using the principle of parsimony, selecting the tree that requires the fewest evolutionary changes to explain the distribution of characters observed.
Key terms in this question
cladogram · shared derived character
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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