Explain how morphological and molecular evidence are each used to classify organisms into taxa, and outline one limitation of using morphological evidence alone.

IB DP Biology Standard Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Morphological evidence involves comparing observable physical features such as body structures, anatomy or form, and grouping organisms that share similar characteristics into the same taxon.
Molecular evidence involves comparing DNA sequences (or amino acid/protein sequences) to determine evolutionary relationships; organisms with more similar sequences are considered more closely related and are placed in the same or closer taxa.
A limitation of using morphological evidence alone is that organisms can share similar structures due to convergent evolution rather than common ancestry, leading to incorrect classification.

Examiner tips

  • Use the word ‘morphological’ and ‘molecular’ explicitly; give a clear example of each type of evidence.
  • State the limitation in one sentence and explain why it matters for classification.
  • Keep the answer concise – 4 marks can be achieved with two short points and one limitation.

Common mistakes

  • Confusing ‘molecular’ with ‘chemical’ evidence or omitting DNA as the key molecule.
  • Failing to mention convergent evolution as the main limitation or giving a vague reason for the limitation.

Mark scheme (4 marks)

  1. Morphological evidence involves comparing observable physical features (such as body structures, anatomy, or form) to group organisms that share similar characteristics into the same taxon.
  2. Molecular evidence involves comparing DNA sequences (or amino acid/protein sequences) to determine evolutionary relationships; organisms with more similar sequences are considered more closely related and are placed in the same or closer taxa.
  3. One limitation of morphological evidence alone: organisms can share similar structures due to convergent evolution rather than common ancestry, leading to incorrect classification.
  4. A further limitation: morphological features can vary within a species (due to age, sex, or environment / phenotypic plasticity), making it difficult to consistently assign organisms to the correct taxon using appearance alone.

Key terms in this question

morphological evidence · molecular evidence

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