Explain why the presence of analogous structures in two species does not indicate that they belong to the same clade, whereas the presence of homologous structures may do so.

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)

Analogous structures arise through convergent evolution – they evolve independently in unrelated lineages because of similar selection pressures, so they do not reflect shared ancestry.

Because they are not inherited from a common ancestor, analogous structures cannot be used as synapomorphies; they cannot group organisms into a clade.

Homologous structures are derived from the same ancestral structure and therefore indicate that the species share a common ancestor.

In cladistics, organisms are grouped on shared derived characters (synapomorphies). Homologous structures that are unique to a group can act as synapomorphies, placing those species in the same clade.

Examiner tips

  • Use the word ‘convergent evolution’ when describing analogous structures. Explain that synapomorphies are derived from a common ancestor. Show the link between homologous structures and shared ancestry. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Confusing analogous with homologous structures. Saying analogous structures prove a clade. Using vague terms like ‘similar’ without explaining convergent evolution.

Mark scheme (4 marks)

  1. Analogous structures arise through convergent evolution, meaning they evolved independently in unrelated lineages in response to similar selection pressures / environments.
  2. Because analogous structures do not reflect shared common ancestry, they cannot be used as synapomorphies / shared derived characters to group organisms into a clade.
  3. Homologous structures are derived from the same ancestral structure and indicate that the species share a common ancestor from which that structure was inherited.
  4. In cladistics, organisms are grouped based on shared derived characters (synapomorphies); homologous structures that are unique to a group can serve as synapomorphies, placing those species in the same clade.

Key terms in this question

analogous structures · homologous structures · clade

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