A population of wild wolves was once found across a vast continent. Around 10 000 years ago, a large glacier advanced and permanently separated the wolves into two isolated groups — one in the north and one in the south. The northern group lived in deep snow environments, while the southern group lived in warm, dry grasslands. After many thousands of years, scientists found that the two groups could no longer interbreed to produce fertile offspring. Explain how the two isolated groups of wolves eventually became two separate species.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 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).

A population of wild wolves was once found across a vast continent. Around 10 000 years ago, a large glacier advanced and permanently separated the wolves into two isolated groups — one in the north and one in the south. The northern group lived in deep snow environments, while the southern group lived in warm, dry grasslands. After many thousands of years, scientists found that the two groups could no longer interbreed to produce fertile offspring.

Model answer (5 marks)

The glacier was a physical barrier that prevented the two wolf groups from interbreeding – this is geographical isolation.

In each isolated group random mutations produced new genetic variants or alleles, giving the populations genetic variation.

Because the northern wolves lived in deep snow and the southern wolves in warm grasslands, natural selection acted on the variation in each environment. Individuals with traits that were advantageous for their specific habitat were more likely to survive and reproduce.

The advantageous alleles were passed on to the next generation, so the frequency of those alleles increased in each population over many generations.

After thousands of years the two populations had accumulated so many genetic differences that they could no longer interbreed to produce fertile offspring. Thus they became separate species – a process known as speciation.

Examiner tips

  • Show the sequence: isolation → mutation → selection → allele frequency change → reproductive isolation
  • Use the exact terms: geographical isolation, random mutations, natural selection, allele frequency, reproductive isolation
  • Keep each point brief but complete
  • Mention the glacier as the barrier

Common mistakes

  • Mixing up the order of events; writing mutation after selection
  • Using vague terms like ‘differences’ instead of ‘alleles’
  • Failing to state that the barrier was the glacier

Mark scheme (5 marks)

  1. The glacier acted as a physical barrier, preventing the two groups from interbreeding / mating with each other (geographical isolation).
  2. Random mutations produced new genetic variants / alleles within each group, creating genetic variation.
  3. In each environment, natural selection acted on the variation — individuals with advantageous characteristics were more likely to survive and reproduce.
  4. The beneficial alleles / characteristics were passed on to offspring, so the frequency of advantageous alleles increased in each population over generations.
  5. Over time, the two populations accumulated so many genetic differences that they could no longer interbreed to produce fertile offspring, so they became separate species (speciation).

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