Natural selection and evolution: how it works, with examples
Understand variation, competition and inheritance, with examples such as peppered moths and antibiotic resistance, and practice questions.
GCSEHigh SchoolAPCurriculum fit: GCSE Biology and Combined Science (evolution and natural selection). US: high school life science (evolution: evidence and natural selection); also AP Biology.
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What is it?
Evolution is the gradual change in the inherited characteristics of a population over many generations. Charles Darwin's theory of natural selection explains how it happens.
Individuals in a species vary. Some variations make an organism better suited to its environment, so it is more likely to survive and reproduce and pass on its alleles. Over time, those helpful features become more common.
Key formulas and ideas
- Natural selection steps: variation, more offspring are born than can survive, competition, the best-adapted survive and breed, they pass on their alleles.
- Adaptation: a feature that helps an organism survive in its environment.
- Evidence for evolution: fossils, DNA comparisons and observed changes such as antibiotic resistance.
Worked examples
Example 1: Peppered moths
Question: Explain why dark peppered moths became more common in polluted cities.
- Moths varied in colour, light or dark.
- Soot darkened the tree bark, so dark moths were better camouflaged.
- Birds ate more light moths, so more dark moths survived and bred.
- The allele for dark colour became more common.
Answer: Natural selection favoured dark moths in polluted areas
Example 2: Antibiotic resistance
Question: How do bacteria become resistant to an antibiotic?
- Random mutation produces a resistant bacterium.
- The antibiotic kills non-resistant bacteria.
- The resistant bacterium survives and reproduces, passing on the resistance allele.
Answer: Resistant bacteria are selected for and increase in number
Example 3: Fossil evidence
Question: Why do fossils support evolution?
- Fossils show organisms that lived in the past.
- Comparing them shows gradual changes over time.
Answer: They show how species have changed
Common mistakes
- Saying that individual organisms change to adapt. Populations change, through selection.
- Saying that 'the strongest' survive. The best adapted to the environment survive.
- Thinking mutations are caused by the need for a feature.
- Writing 'survival of the fittest' without explaining inheritance.
Practice questions
Frequently asked questions
Is evolution 'only a theory'?
In science a theory is a well-tested explanation supported by large amounts of evidence.
What is speciation?
The formation of a new species, which can happen when populations become isolated and diverge over time.
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KS3GCSEMiddle SchoolHigh SchoolSources and review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.