Explain how cladistics differs from traditional Linnaean classification in the way it groups organisms, and outline one advantage of using cladistics in modern taxonomy.

IB DP Biology Higher 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)

Cladistics groups organisms on the basis of shared derived traits (synapomorphies) that indicate common evolutionary ancestry, whereas traditional Linnaean classification groups them mainly on overall morphological similarity and physical features.

Cladistics therefore produces monophyletic groups (clades) that contain an ancestor and all its descendants, while Linnaean groups can be paraphyletic or polyphyletic.

An advantage of cladistics is that it more accurately reflects true evolutionary history, and it can incorporate molecular or genetic data (e.g. DNA sequence comparisons) to refine groupings, allowing re‑classification when morphology is misleading such as in cases of convergent evolution.

Examiner tips

  • Use the terms "synapomorphies" and "monophyletic" to hit the key points
  • Show the contrast with Linnaean "morphological similarity" and note the possibility of paraphyly/polyphyly
  • Mention the use of DNA data as a concrete advantage

Common mistakes

  • Confusing cladistics with phylogenetic trees without mentioning synapomorphies
  • Saying Linnaean groups are always monophyletic
  • Failing to note the role of genetic data in modern cladistics

Mark scheme (4 marks)

  1. Cladistics groups organisms based on shared derived characteristics (synapomorphies) / shared evolutionary ancestry, whereas Linnaean classification groups organisms based on overall morphological similarity / shared physical features.
  2. Cladistics produces monophyletic groups (clades) — each group contains an ancestor and all of its descendants — whereas Linnaean groups can be paraphyletic or polyphyletic.
  3. One advantage: cladistics more accurately reflects true evolutionary history / phylogeny of organisms.
  4. Cladistics can incorporate molecular / genetic data (e.g. DNA sequence comparisons) to construct or refine groupings, allowing reclassification when morphology is misleading (e.g. convergent evolution).

Key terms in this question

cladistics · Linnaean classification

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