Explain how geographic isolation can lead to the formation of two new species from a single ancestral population.

IB DP Biology Standard 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).

Model answer (4 marks)

Geographic isolation stops gene flow between the separated populations.
Each population is then exposed to different selection pressures and/or random mutation, so allele frequencies change independently.
Over time, genetic differences accumulate until individuals from the two populations can no longer interbreed to produce fertile offspring.
When reproductive isolation is established, the two groups are recognised as separate species (biological species concept).

Examiner tips

Use the term ‘geographic isolation’ first; link it to ‘no gene flow’.
Show the chain: isolation → independent evolution → reproductive isolation → new species.
Use the exact wording from the mark scheme to hit each point.

Common mistakes

Confusing geographic isolation with ecological isolation; they are not the same.
Failing to mention that reproductive isolation is the final step for species formation.
Using vague terms like ‘differences’ without specifying allele frequency changes or reproductive barriers.

Mark scheme (4 marks)

Geographic isolation prevents gene flow between the separated populations
Each population is subject to different selection pressures and/or random mutation, so allele frequencies change independently in each population
Over time, genetic differences accumulate to the point where individuals from the two populations are no longer able to interbreed to produce fertile offspring
Once reproductive isolation is established, the two groups are defined as separate species (biological species concept)

Key terms in this question

geographic isolation