# Explain how the reclassification of birds within the reptile clade illustrates the principle that all taxa recognised in cladistic classification must be monophyletic.

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

## Mark scheme (4 marks)

1. A monophyletic group (clade) contains an ancestor and ALL of its descendants.
2. Traditional Reptilia excluded birds but included their ancestor, making it paraphyletic (not all descendants included).
3. Molecular and morphological evidence shows birds (Aves) share a more recent common ancestor with crocodilians than crocodilians do with lizards/snakes, placing birds firmly within the reptile clade.
4. Cladistics therefore requires either including birds within Reptilia (making Reptilia monophyletic) or abandoning Reptilia as a valid taxon, since a group that excludes some descendants of its own ancestor cannot be accepted in a cladistic framework.

## Key terms

- [monophyletic](https://www.gradenine.co.uk/glossary/monophyletic)
- [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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-reclassification-of-birds-deaa1589) · Published by Druglandscape Ltd.