Explain how molecular evidence from gene sequencing has changed our understanding of the classification of living organisms, with reference to the three-domain system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Gene sequencing, such as of 16S/18S rRNA genes, provides a precise comparison of nucleotide sequences between organisms, allowing us to assess evolutionary relatedness.
Molecular data showed that Archaea are as distinct from Bacteria as they are from Eukaryota, which led to replacing the five‑kingdom system with the three‑domain system (Bacteria, Archaea, Eukarya).
Because morphology can be misleading (analogous features arise independently), molecular evidence can reveal that organisms with similar appearance are not closely related, correcting classifications based on appearance alone.
Using sequence data, scientists construct more accurate phylogenetic trees or cladograms that reflect true evolutionary history and common ancestry of organisms.
Molecular data showed that Archaea are as distinct from Bacteria as they are from Eukaryota, which led to replacing the five‑kingdom system with the three‑domain system (Bacteria, Archaea, Eukarya).
Because morphology can be misleading (analogous features arise independently), molecular evidence can reveal that organisms with similar appearance are not closely related, correcting classifications based on appearance alone.
Using sequence data, scientists construct more accurate phylogenetic trees or cladograms that reflect true evolutionary history and common ancestry of organisms.
Examiner tips
- Use the word ‘gene sequencing’ and give an example (16S/18S rRNA).
- Show the link between Archaea, Bacteria and Eukarya to justify the three‑domain system.
- Explain how morphology can be misleading and how DNA data corrects this.
- Mention phylogenetic trees/cladograms as the outcome of molecular data.
Common mistakes
- Confusing the five‑kingdom system with the three‑domain system; not stating the replacement clearly.
- Failing to mention that 16S/18S rRNA genes are the typical markers used.
- Overlooking the point that similar morphology does not imply close relation.
Mark scheme (4 marks)
- Gene sequencing (e.g. of 16S/18S rRNA genes) allows comparison of nucleotide sequences between organisms to determine evolutionary relatedness
- 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)
- Molecular evidence can reveal that organisms with similar morphology (analogous features) are not closely related, overturning classifications based on appearance alone
- Molecular data allows construction of more accurate phylogenetic trees / cladograms, better reflecting true evolutionary history (common ancestry) of organisms
Key terms in this question
three-domain system · gene sequencing
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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