# Explain why the presence of analogous structures in two species does not indicate that they belong to the same clade, whereas the presence of homologous structures may do so.

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

## Mark scheme (4 marks)

1. Analogous structures arise through convergent evolution, meaning they evolved independently in unrelated lineages in response to similar selection pressures / environments.
2. Because analogous structures do not reflect shared common ancestry, they cannot be used as synapomorphies / shared derived characters to group organisms into a clade.
3. Homologous structures are derived from the same ancestral structure and indicate that the species share a common ancestor from which that structure was inherited.
4. In cladistics, organisms are grouped based on shared derived characters (synapomorphies); homologous structures that are unique to a group can serve as synapomorphies, placing those species in the same clade.

## Key terms

- [analogous structures](https://www.gradenine.co.uk/glossary/analogous-structures)
- [homologous structures](https://www.gradenine.co.uk/glossary/homologous-structures)
- [clade](https://www.gradenine.co.uk/glossary/clade)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "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/explain-why-the-presence-of-analogous-7dd38a89) · Published by Druglandscape Ltd.