Explain how natural selection can lead to a population of organisms becoming better adapted to their environment over many generations.

AQA A-Level Biology (7402) — 3.4.4 Genetic Diversity and Adaptation · Explain · 5 marks · View as Markdown

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

A population of moths lives in a woodland area. The bark of the trees is pale in colour. A dark-coloured variant of the moth occasionally appears due to mutation.

Model answer (5 marks)

1. There is variation in the population – some moths are pale, some are dark.
2. Predators are less likely to spot pale moths against the pale bark, so pale moths experience lower predation.
3. Pale moths are therefore more likely to survive to reproductive age.
4. They pass on their alleles for pale colouration to their offspring.
5. Over many generations the proportion of pale moths increases, making the population better adapted to the environment.

Examiner tips

Use the exact wording from the mark scheme – mention variation, selective pressure, survival, inheritance, change over generations.
Show the causal chain clearly – each point should build on the previous one.
Keep sentences short and to the point to fit the 5‑mark limit.

Common mistakes

Mixing up which colour is selected for – students sometimes say dark moths are favoured.
Failing to mention the role of inheritance – saying moths simply survive without passing on traits.
Using vague terms like ‘better suited’ without linking to specific selective pressure.

Mark scheme (5 marks)

There is variation within the population (e.g. some moths are pale, some are dark)
Predators are less likely to spot pale moths against pale bark (selective pressure / predation acting as selection pressure)
Pale moths are more likely to survive and reproduce
Pale moths pass on their alleles / genes for pale colouration to offspring
Over many generations the proportion of pale moths in the population increases, so the population becomes better adapted

Key terms in this question

natural selection