Explain how a cladogram can be used to identify the most recent common ancestor of two species, and outline what the position of that ancestor on the cladogram indicates about the evolutionary relationship between those species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Trace the two species back to the first node where their branches meet – this node is the most recent common ancestor. The closer this node is to the tips, the more recent the ancestor and the closer the relationship. All lineages that descend from that node form a clade, a monophyletic group that shares that ancestor. The branching pattern shows that the two species diverged from a common ancestral population, not that one evolved directly from the other.
Examiner tips
- Use the word ‘node’ to show the ancestor, mention ‘closest node’ for closeness, state ‘clade/monophyletic group’, and explain divergence rather than direct descent
Common mistakes
- Confusing the node with the species itself, or saying the ancestor is the species; ignoring the clade concept; claiming one species evolved from the other instead of diverging from a common ancestor.
Mark scheme (4 marks)
- The most recent common ancestor is identified by tracing the lineages of both species back to the node where they first converge/meet on the cladogram.
- The closer (more recent) the node, the more closely related the two species are / they share a more recent common ancestor.
- All species that share that node (and all descendant lineages from it) form a clade / monophyletic group, meaning they all descended from that single common ancestor.
- The branching pattern shows the sequence of divergence events, indicating that the two species diverged from a common ancestral population rather than one species giving rise directly to the other.
Key terms in this question
cladogram · most recent common ancestor
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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