Distinguish between analogous structures and homologous structures, using one named example of each to support your answer.

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

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

Model answer (4 marks)

Homologous structures are parts of different organisms that have a common evolutionary origin and share a similar underlying anatomy, even if their functions differ. For example, the pentadactyl limb of the human arm, the whale flipper and the bat wing all have the same basic bone arrangement.

Analogous structures are parts that perform a similar function but do not share a common ancestral structure; they arise independently through convergent evolution. An example is the wings of a bird and the wings of an insect, both used for flight but evolved separately.

Examiner tips

  • Define each term clearly before giving examples. Use the word "homologous" and "analogous" exactly. Include the key point about common ancestry for homologous and independent origin for analogous. Give one correct named example for each.
  • common_mistakes
  • :
  • Mixing up the definitions (e.g. saying analogous structures share ancestry). Using an example that is actually homologous for the analogous part, such as the forelimbs of a cat and a dog. Omitting the word "pentadactyl" or the specific example names.

Mark scheme (4 marks)

  1. Homologous structures share a common evolutionary origin / are derived from the same ancestral structure, even if they differ in function.
  2. Analogous structures perform a similar function but do not share a common evolutionary origin / arose independently through convergent evolution.
  3. A named, correct example of homologous structures, e.g. the pentadactyl limb found in humans (arm), whales (flipper), and bats (wing) — same underlying bone arrangement but different functions.
  4. A named, correct example of analogous structures, e.g. the wings of a bird and an insect — both used for flight but evolved independently with no common structural ancestry.

Key terms in this question

analogous structures · homologous structures

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