Explain how variation within a population of organisms can lead to some individuals surviving long enough to reproduce whilst others do not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a population of peppered moths, individuals show variation in wing colour, ranging from pale to very dark. In areas where tree bark is darkened by pollution, darker moths are more common.
Model answer (4 marks)
Variation exists within the population, with moths showing different wing colours.
Some individuals have a phenotype that is better suited to the environment – e.g. darker moths blend with polluted bark.
These better‑adapted moths are more likely to survive and reproduce, while less well‑adapted moths are more likely to be eaten or die before reproducing.
The advantageous allele for dark colour is passed on to offspring, so its frequency increases in the population over many generations.
Some individuals have a phenotype that is better suited to the environment – e.g. darker moths blend with polluted bark.
These better‑adapted moths are more likely to survive and reproduce, while less well‑adapted moths are more likely to be eaten or die before reproducing.
The advantageous allele for dark colour is passed on to offspring, so its frequency increases in the population over many generations.
Examiner tips
- Use the exact terms ‘variation’, ‘phenotype’, ‘advantage’, ‘survive’, ‘reproduce’, ‘allele’, ‘frequency’
- Show the causal chain: variation → advantage → survival → reproduction → allele frequency change
- Keep each point brief and directly linked to the mark scheme
Common mistakes
- Confusing ‘variation’ with ‘mutation’; not stating that variation is within the population
- Failing to mention that the advantageous allele increases in frequency over generations
- Using vague language such as ‘better suited’ without linking to survival and reproduction
Mark scheme (4 marks)
- Variation exists within the population / individuals in the population have different phenotypes / different wing colours
- Some individuals have a characteristic / phenotype that is better suited to the environment / gives an advantage
- Better-adapted individuals are more likely to survive and reproduce / less well-adapted individuals are more likely to be eaten / die before reproducing
- The advantageous allele is passed on to offspring / the frequency of the advantageous allele increases in the population over many generations
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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