Explain how the reclassification of birds within the reptile clade illustrates the principle that all taxa recognised in cladistic classification must be monophyletic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A monophyletic group (clade) contains an ancestor and ALL of its descendants.
Traditional Reptilia excluded birds but included their ancestor, making it paraphyletic (not all descendants included).
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.
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.
Traditional Reptilia excluded birds but included their ancestor, making it paraphyletic (not all descendants included).
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.
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.
Examiner tips
- Use the definition of a monophyletic group explicitly. Show the traditional error (paraphyly) and the evidence that corrects it. Explain the two possible solutions to maintain a monophyletic Reptilia. Keep answer concise to fit 4 marks.
Common mistakes
- Confusing paraphyletic with polyphyletic. Failing to mention the recent common ancestor with crocodilians. Using vague terms like "birds are reptiles" without explaining monophyly.
Mark scheme (4 marks)
- A monophyletic group (clade) contains an ancestor and ALL of its descendants.
- Traditional Reptilia excluded birds but included their ancestor, making it paraphyletic (not all descendants included).
- 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.
- 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 in this question
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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