# Distinguish between an outgroup and an ingroup in the context of cladistic analysis, and explain the role of the outgroup in identifying synapomorphies.

> IB DP Biology Higher Level (2023 syllabus) — A3.2 Classification and cladistics · Distinguish / Explain · 4 marks

## Mark scheme (4 marks)

1. The ingroup comprises the taxa under study / the organisms whose evolutionary relationships are being investigated.
2. 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.
3. 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.
4. 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

- [outgroup](https://www.gradenine.co.uk/glossary/outgroup)
- [ingroup](https://www.gradenine.co.uk/glossary/ingroup)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Distinguish / Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/distinguish-between-an-outgroup-and-an-a4d3e485) · Published by Druglandscape Ltd.