A population of mountain hares lives in a cold northern region. The hares have white fur, which acts as camouflage against snow. Over many generations, a small group of hares migrates southward to a warmer region with less snowfall. After thousands of years, scientists find that the southern hares can no longer interbreed with the northern hares to produce fertile offspring. Explain how the southern population of hares could have evolved into a new species.

Pearson Edexcel A-Level Biology A (9BI0) — 3.4 Evolution and speciation · Explain · 5 marks · View as Markdown

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

Mountain hares in the northern region have white fur all year round. The southern region has fewer months of snow cover and a greater variety of predators that hunt by sight.

Model answer (5 marks)

The two populations become geographically isolated, so interbreeding is prevented.
Random mutations arise in the DNA of individuals in both groups, creating genetic variation.
In the southern group, individuals with alleles that suit the warmer, less snowy environment (for example, darker or brown fur) have a survival advantage and are better adapted.
These better‑adapted individuals are more likely to survive and reproduce, passing their advantageous alleles on to their offspring – this is natural selection.
After many generations the allele frequencies in the two populations diverge so far that they can no longer produce fertile offspring, so the southern group has become a new species.

Examiner tips

  • Mention geographic isolation first, as it is the key to all subsequent steps.
  • Use the exact terms ‘random mutations’, ‘allele frequencies’, ‘natural selection’ and ‘speciation’ – the mark scheme rewards precise terminology.
  • Show the causal chain: isolation → variation → selection → divergence → speciation.
  • Keep the answer concise – 5 marks, so one sentence per point is sufficient.

Common mistakes

  • Confusing ‘adaptation’ with ‘speciation’ – students sometimes stop at adaptation and forget to mention reproductive isolation.
  • Using vague language such as ‘differences’ instead of ‘allele frequencies’ or ‘natural selection’ – examiners look for specific terms.
  • Failing to state that the two populations are geographically isolated – this is the first point in the mark scheme.

Mark scheme (5 marks)

  1. The two populations are geographically isolated / separated, preventing interbreeding between them
  2. Random mutations occur in the DNA of individuals in both populations, producing genetic variation
  3. In the southern population, individuals with alleles better suited to the warmer, less snowy environment (e.g. darker/brown fur) have a survival advantage / are better adapted
  4. These better-adapted individuals are more likely to survive and reproduce, passing their alleles on to offspring (natural selection)
  5. Over many generations the allele frequencies in the two populations become so different that they can no longer interbreed to produce fertile offspring, so they are now different species (speciation)

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