Explain how natural selection leads to a population of animals becoming better adapted to their environment over many generations.

Pearson Edexcel A-Level Biology A (9BI0) — 2.6 Adaptation and natural selection · Explain · 5 marks · View as Markdown

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

In a population of moths, individuals vary in their body colour. Some moths are pale and some are darker. Birds hunt moths by sight. In an environment where tree bark is dark, darker moths are less easily spotted by predators.

Model answer (5 marks)

Variation in body colour exists in the moth population.
Darker moths are better camouflaged against dark bark and are less likely to be seen by birds.
Because they are less likely to be eaten, darker moths survive to reproduce more often than pale moths.
They pass on the alleles for darker colouring to their offspring.
Over many generations the proportion of darker moths increases, making the population better adapted to the dark environment.

Examiner tips

Show the step‑by‑step causal chain: variation → differential survival → differential reproduction → change in allele frequency.
Use the exact terms: variation, natural selection, survival, reproduction, allele, proportion.
Keep the answer concise – 5 points, one sentence per point.
Mention that the change occurs over many generations to justify the long‑term effect.

Common mistakes

Failing to mention that variation is necessary for selection to act.
Using vague language such as "more common" instead of "proportion increases".
Omitting the role of reproduction in passing on the trait.

Mark scheme (5 marks)

There is variation in the population (e.g. in body colour)
Darker moths are better camouflaged / less likely to be seen by predators on dark bark
Darker moths are more likely to survive (and reproduce) / pale moths are more likely to be eaten
Darker moths pass on the alleles / genes for darker colouring to their offspring
Over many generations the proportion of darker moths in the population increases

Key terms in this question

natural selection