A population of beetles lives on a large island. Over thousands of years, the island splits into two separate landmasses due to rising sea levels, dividing the beetle population. After many generations, the two groups of beetles can no longer interbreed to produce fertile offspring. Describe how genetic variation and natural selection led to this outcome.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A population of beetles lives on a large island. Over thousands of years, rising sea levels split the island into two separate landmasses, permanently separating the beetle population into two isolated groups. After many generations, the two groups of beetles can no longer interbreed to produce fertile offspring.
Model answer (5 marks)
Random mutations in the DNA base sequence create genetic variation within each isolated beetle population.
Because the two landmasses have different environments, each population experiences distinct selection pressures.
Alleles that are advantageous in one environment become more common in that population through natural selection, while disadvantageous alleles are removed.
Over many generations the allele frequencies diverge, producing genetic differences between the two groups.
Eventually the accumulated differences prevent the two groups from producing fertile offspring, so they become separate species (speciation).
Because the two landmasses have different environments, each population experiences distinct selection pressures.
Alleles that are advantageous in one environment become more common in that population through natural selection, while disadvantageous alleles are removed.
Over many generations the allele frequencies diverge, producing genetic differences between the two groups.
Eventually the accumulated differences prevent the two groups from producing fertile offspring, so they become separate species (speciation).
Examiner tips
- Use the word "mutations" and "alleles" to show knowledge of genetics. Explain that different environments create different selection pressures. Show the causal chain: mutation → variation → selection → divergence → speciation. Mention that reproductive isolation is the final outcome.
Common mistakes
- Confusing mutation with natural selection; they are separate processes. Failing to state that the two populations are isolated and therefore cannot interbreed. Using vague terms like "changes" instead of specific genetic concepts such as "alleles" or "genetic variation."
Mark scheme (5 marks)
- Random mutations occur in the DNA base sequence of individuals in each population
- This creates genetic variation between individuals within each separated population
- The two populations experience different selection pressures / environments, so different alleles are advantageous in each group
- Individuals with beneficial alleles are more likely to survive and reproduce, passing those alleles to offspring (natural selection)
- Over many generations the two populations become so genetically different that they can no longer interbreed to produce fertile offspring, forming two separate species (speciation)
Key terms in this question
genetic variation · natural selection
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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