Distinguish between an outgroup and an ingroup in the context of cladistic analysis, and explain the role of the outgroup in identifying synapomorphies.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The ingroup consists of the taxa whose relationships are being studied.
The outgroup is a taxon that diverged earlier than the ingroup and is more distantly related to all ingroup members.
By comparing the ingroup to the outgroup we can recognise plesiomorphic states – traits shared with the outgroup are likely ancestral.
Synapomorphies are traits found in two or more ingroup members but absent in the outgroup, indicating shared derived character and defining a monophyletic clade.
The outgroup is a taxon that diverged earlier than the ingroup and is more distantly related to all ingroup members.
By comparing the ingroup to the outgroup we can recognise plesiomorphic states – traits shared with the outgroup are likely ancestral.
Synapomorphies are traits found in two or more ingroup members but absent in the outgroup, indicating shared derived character and defining a monophyletic clade.
Examiner tips
- Use the words 'ingroup' and 'outgroup' explicitly; explain their relative positions in the tree.
- Show that the outgroup provides a reference for ancestral states, then state how traits absent in the outgroup but present in the ingroup are synapomorphies.
Common mistakes
- Confusing ingroup with outgroup or vice‑versa.
- Failing to explain that the outgroup helps identify ancestral versus derived traits.
Mark scheme (4 marks)
- The ingroup comprises the taxa under study / the organisms whose evolutionary relationships are being investigated.
- The outgroup is a taxon that is more distantly related to all members of the ingroup / diverged from the ingroup before the ingroup taxa diverged from each other.
- Comparing the ingroup to the outgroup allows ancestral (plesiomorphic) character states to be identified, because traits shared between the outgroup and some ingroup members are likely ancestral.
- Synapomorphies (shared derived characters) are identified as traits present in two or more ingroup members but absent in the outgroup, indicating shared common ancestry within the ingroup / used to define monophyletic clades.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Distinguish / Explain" question →
- Decode the mark scheme abbreviations →
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