Explain why cladistics may result in the reclassification of taxa that were previously grouped together under traditional Linnaean taxonomy.

IB DP Biology Higher 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)

1. Traditional Linnaean taxonomy groups organisms on overall morphological similarity, which can include analogous traits that are not inherited from a common ancestor.
2. Cladistics, by contrast, groups organisms only on shared derived characters (synapomorphies) that are inherited from a common ancestor.
3. When two organisms share a character because of convergent evolution rather than common ancestry, cladistics will place them in separate clades, whereas Linnaean classification may have grouped them together.
4. Because cladistics recognises only monophyletic groups, any taxon found to be paraphyletic or polyphyletic must be re‑classified to reflect true evolutionary relationships.

Examiner tips

  • Use the word ‘synapomorphy’ to show understanding of cladistic criteria.
  • Show contrast between analogous and homologous traits.
  • Explain that reclassification is required for paraphyletic/polyphyletic taxa.
  • Keep each point concise and directly linked to the marks given.

Common mistakes

  • Confusing homologous with analogous traits; students often say ‘similarity’ without specifying the type of similarity.
  • Failing to mention that cladistics only accepts monophyletic groups, leading to incomplete explanations.
  • Using vague language such as ‘different groups’ instead of specifying clades or monophyly.

Mark scheme (4 marks)

  1. Traditional Linnaean classification groups organisms based on overall morphological similarity / overall resemblance, which can include analogous (homoplasious) characters.
  2. Cladistics groups organisms exclusively by shared derived characters (synapomorphies) that indicate common ancestry.
  3. If two organisms share a character due to convergent evolution rather than common ancestry, cladistics would place them in different clades, whereas Linnaean classification may have grouped them together.
  4. Cladistics only recognises monophyletic groups (clades), so any taxon found to be paraphyletic or polyphyletic must be reclassified to reflect true evolutionary / phylogenetic relationships.

Key terms in this question

cladistics · Linnaean taxonomy

Related

More Classification and cladistics questions

▶ Try answering this question with AI marking (free) →