Natural Selection IGCSE Biology 0610

Natural Selection: The Mechanism of Evolution

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Natural selection occurs because individuals within a species show variation. Those with features best suited to the environment are more likely to survive, compete for resources, reproduce and pass on their alleles to the next generation. Over time, the frequency of advantageous alleles increases.

Natural selection is often poorly explained because students describe the outcome ("the organism adapted") without explaining the mechanism. The answer must follow a strict sequence: variation, selection pressure, differential survival, reproduction and allele frequency change.

The step-by-step mechanism

The mechanism of natural selection can be described in a clear sequence:

1. Within a population, individuals show genetic variation due to mutation, meiosis and sexual reproduction.
2. The environment presents selection pressures such as predation, disease, competition for food, water and mates.
3. Individuals with characteristics better suited to the environment are more likely to survive (survival of the fittest).
4. These individuals are more likely to reproduce and pass on the alleles for the advantageous characteristics to their offspring.
5. Over many generations, the frequency of the advantageous alleles increases in the population.

Key points examiners look for

Examiners specifically look for these points: that variation exists before the selection pressure is applied, that the selection is about differential survival and reproduction (not organisms deliberately changing), and that the change occurs over many generations, not within a single organism's lifetime.

Never write that organisms "choose to adapt", "try to change" or "evolve in response to a need". Natural selection acts on existing variation; it does not create variation in response to a challenge. The variation must already be present for selection to act upon it.

Examples of natural selection

Peppered moths: Before industrial pollution, light-coloured moths were camouflaged against pale tree bark and survived better. When soot darkened the bark, dark-coloured moths were better camouflaged and survived predation more often. The frequency of the dark allele increased in polluted areas. When pollution reduced, the light form became more common again.

Antibiotic-resistant bacteria: Some bacteria carry mutations giving resistance to an antibiotic. When the antibiotic is applied, non-resistant bacteria die. Resistant bacteria survive, reproduce and pass on the resistance allele, increasing its frequency. This is natural selection with the antibiotic as the selection pressure.

Natural selection vs selective breeding

In natural selection, the environment determines which organisms survive and reproduce. No human intention is involved. In selective breeding (artificial selection), humans choose organisms with desired traits to breed together.

Both processes change allele frequencies over generations. The difference is the selection agent: nature in one case and humans in the other. Natural selection tends to maintain or increase genetic diversity, while selective breeding tends to reduce it.

Try a focused question (4 marks)

Explain how natural selection could lead to an increase in the proportion of dark-coloured moths in a polluted forest.

Show the answer and marking guidance

There is genetic variation in moth colour: some are light and some are dark.
In a polluted forest, dark moths are better camouflaged on soot-covered trees and are less likely to be eaten by predators.
Dark moths survive longer and are more likely to reproduce.
They pass on the allele for dark colour to their offspring, so over generations the proportion of dark moths increases.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying organisms "adapt" or "evolve" as if it is a deliberate choice.
Forgetting that variation must exist before selection acts.
Describing the outcome without explaining the mechanism.
Claiming natural selection occurs within a single generation.