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

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 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 beetles, individuals vary in the colour of their bodies. Some beetles are dark brown and others are pale green. The beetles live on green leaves and are hunted by birds.

Model answer (5 marks)

There is variation in the population – some beetles are dark brown, others pale green.
Pale green beetles are better camouflaged on green leaves and are less likely to be eaten by birds, so they experience less predation.
Because they survive longer, pale green beetles are more likely to reproduce.
The alleles that give the pale green colour are therefore passed on to their offspring.
Over many generations the proportion of pale green beetles in the population increases, making the population better adapted to its environment.

Examiner tips

Show the step‑by‑step chain of natural selection: variation → differential survival → differential reproduction → change in allele frequency.
Use the exact terms: variation, selection pressure, advantageous allele, proportion of the population.
Keep the answer concise – 5 points, one sentence per point.
Use UK spelling (e.g. ‘colour’).

Common mistakes

Failing to mention that the advantageous allele is passed to offspring.
Using vague words like ‘better’ without linking to camouflage or predation.
Writing the answer as a paragraph instead of separate points, losing clarity.

Mark scheme (5 marks)

There is variation in the population (e.g. some beetles are dark brown and some are pale green)
Pale green beetles are better camouflaged on leaves and are less likely to be eaten by birds (selection pressure / predation)
Better-adapted (pale green) beetles are more likely to survive and reproduce
The advantageous alleles / genes for pale green colour are passed on to offspring
Over many generations, the proportion of pale green beetles in the population increases

Key terms in this question

natural selection · population