Explain how the concept of a changing environment contributes to natural selection acting on a population over successive generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A population of ground-dwelling beetles lives in a habitat where the soil colour shifts gradually from pale sandy loam to darker clay over several decades due to changing drainage patterns.
Model answer (4 marks)
Variation exists within the beetle population in traits such as body colouration, arising from random mutations and genetic recombination.
As the soil darkens, beetles with darker colouration have better camouflage and are less likely to be detected and eaten by predators, giving them a selective advantage and higher relative fitness.
Individuals with the advantageous phenotype survive longer and reproduce more, passing the alleles for darker colouration to their offspring at a higher frequency – this is differential reproductive success.
Over successive generations the allele frequency of darker‑colouration alleles increases in the population, shifting the population’s mean phenotype and demonstrating evolution by natural selection.
As the soil darkens, beetles with darker colouration have better camouflage and are less likely to be detected and eaten by predators, giving them a selective advantage and higher relative fitness.
Individuals with the advantageous phenotype survive longer and reproduce more, passing the alleles for darker colouration to their offspring at a higher frequency – this is differential reproductive success.
Over successive generations the allele frequency of darker‑colouration alleles increases in the population, shifting the population’s mean phenotype and demonstrating evolution by natural selection.
Examiner tips
- Use the word "explain" to describe the causal chain: variation → differential survival → differential reproduction → change in allele frequency.
- Mention the changing environment (soil colour) as the selective pressure.
- Show the link between higher fitness and increased allele frequency.
- Keep the answer concise – 4 marks allow a brief, logical sequence.
Common mistakes
- Failing to mention that the soil colour change is the selective pressure.
- Confusing fitness with survival only, ignoring reproductive success.
- Using vague terms like "better suited" without linking to allele frequency change.
Mark scheme (4 marks)
- Variation exists within the beetle population in traits such as body colouration, arising from random mutations and genetic recombination.
- As soil darkens, darker-coloured beetles have greater camouflage and are less likely to be detected and eaten by predators, giving them a selective advantage / higher relative fitness.
- Individuals with the advantageous phenotype survive longer and reproduce more, passing the alleles for darker colouration to offspring at a higher frequency — this is differential reproductive success.
- Over successive generations, the allele frequency of darker-colouration alleles increases in the population, shifting the population's mean phenotype — illustrating evolution by natural selection.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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