Explain how the classification of species can change over time as new evidence becomes available, using the relationship between morphological and molecular data as examples.

IB DP Biology Standard Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Traditional classification relied on morphological similarities, grouping organisms that looked alike.
Molecular data (DNA/RNA or protein sequences) can uncover relationships that morphology misses.
Convergent evolution can make unrelated species look similar, misleading morphological classification.
When molecular evidence contradicts morphology, taxa are re‑classified to reflect true evolutionary relationships.

Examiner tips

  • Use the word ‘morphological’ and ‘molecular’ explicitly; link each point to a reason for change.
  • Show the cause (convergent evolution) and the effect (re‑classification).

Common mistakes

  • Confusing ‘morphology’ with ‘genetics’ or omitting the role of convergent evolution.
  • Giving only one example or not linking the evidence to a change in classification.

Mark scheme (4 marks)

  1. Traditional classification was based on morphological (physical/structural) similarities between organisms
  2. Molecular data (DNA/RNA sequences or amino acid sequences) can reveal evolutionary relationships not apparent from morphology alone
  3. Convergent evolution can cause unrelated organisms to appear morphologically similar, leading to incorrect classification
  4. When molecular evidence contradicts morphological groupings, taxa may be reclassified / classification is revised to reflect true evolutionary (phylogenetic) relationships

Key terms in this question

molecular data

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