# Explain how molecular evidence from gene sequencing has changed our understanding of the classification of living organisms, with reference to the three-domain system.

> IB DP Biology Higher Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks

## Mark scheme (4 marks)

1. Gene sequencing (e.g. of 16S/18S rRNA genes) allows comparison of nucleotide sequences between organisms to determine evolutionary relatedness
2. Molecular evidence revealed that Archaea are as distinct from Bacteria as they are from Eukaryota, leading to the replacement of the five-kingdom system with the three-domain system (Bacteria, Archaea, Eukarya)
3. Molecular evidence can reveal that organisms with similar morphology (analogous features) are not closely related, overturning classifications based on appearance alone
4. Molecular data allows construction of more accurate phylogenetic trees / cladograms, better reflecting true evolutionary history (common ancestry) of organisms

## Key terms

- [three-domain system](https://www.gradenine.co.uk/glossary/three-domain-system)
- [gene sequencing](https://www.gradenine.co.uk/glossary/gene-sequencing)

## 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-molecular-evidence-from-gene-5d45e3d1) · Published by Druglandscape Ltd.