Explain how character displacement can occur when two species with overlapping ecological niches come into secondary contact after a period of geographic isolation.

IB DP Biology Standard Level (2023 syllabus) — B4.2 Ecological niches · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When two species come into secondary contact, their niches overlap, leading to inter‑specific competition for shared resources.
Individuals whose phenotype/traits differ most from the competing species experience less competition and therefore have greater fitness, survival and reproductive success.
Natural selection therefore favours divergence of the trait/character in the zone of sympatry, causing the two populations to become more different from each other over generations.
The outcome is greater niche differentiation/partitioning between the two species in sympatry compared with allopatric populations, reducing competition and allowing coexistence.

Examiner tips

  • Use the word ‘explain’ to describe the sequence of events; include competition, selection and divergence.
  • Show that the change is driven by fitness differences in the sympatric zone.
  • Mention that the result is reduced competition and coexistence.

Common mistakes

  • Confusing primary with secondary contact; not emphasising the role of competition.
  • Failing to link phenotype differences to fitness and selection.
  • Giving a generic statement about ‘evolution’ without describing the process of character displacement.

Mark scheme (4 marks)

  1. When two species come into secondary contact, their niches overlap, leading to interspecific competition for shared resources.
  2. Individuals whose phenotype/traits differ most from the competing species experience less competition and therefore have greater fitness/survival and reproductive success.
  3. Natural selection therefore favours divergence of the trait/character in the zone of sympatry, causing the two populations to become more different from each other over generations.
  4. The outcome is greater niche differentiation/partitioning between the two species in sympatry compared with allopatric populations, reducing competition and allowing coexistence.

Key terms in this question

character displacement · ecological niche

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