Explain how convergent evolution provides evidence that natural selection, rather than chance alone, drives evolutionary change.

IB DP Biology Higher Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks · View as Markdown

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

Convergent evolution describes the independent evolution of similar traits in distantly related lineages. Classic examples include the streamlined body form of sharks (Chondrichthyes), ichthyosaurs (extinct reptiles), and dolphins (Mammalia), and the camera-type eye found in both vertebrates and cephalopod molluscs.

Model answer (4 marks)

Convergent evolution occurs when distantly related organisms independently evolve similar structures.
The similarity arises because both lineages face the same selective pressures in analogous environments.
If evolution were driven only by chance, the probability of the same complex trait appearing twice would be negligible.
Thus the repeated, independent appearance of the same adaptive solution shows that natural selection consistently favours particular phenotypes, proving it is a directional, non‑random force.

Examiner tips

Use the word ‘convergent evolution’ early. Explain that similar traits arise from similar selective pressures. Contrast this with the improbability of chance alone. Show that repeated independent origins prove selection is non‑random.

Common mistakes

Confusing convergent with divergent evolution. Failing to mention the role of selective pressures. Treating chance as a sufficient explanation for complex traits.

Mark scheme (4 marks)

Convergent evolution occurs when distantly related / phylogenetically unrelated organisms independently evolve similar structures or traits
The similar traits arise because both lineages experience the same / comparable selective pressures from analogous environments or ecological niches
If evolution were driven by chance (random mutation / genetic drift) alone, the probability of the same complex trait arising independently in unrelated lineages would be negligibly small
The repeated, independent origin of the same adaptive solution therefore demonstrates that natural selection consistently favours particular phenotypes when selective pressures are equivalent, confirming selection as a directional, non-random mechanism of evolutionary change

Key terms in this question

convergent evolution · natural selection