How to Revise IGCSE Biology Topic 18: Variation and Selection
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Distinguish genetic and environmental variation. Know the four steps of natural selection in order. Understand selective breeding advantages and disadvantages (especially reduced genetic diversity).
Topic 18 links genetics to evolution. Key areas: types of variation (genetic vs environmental vs both), the four steps of natural selection, selective breeding, and adaptations. The natural selection process is a guaranteed exam topic.
Priority 1: Types of variation
Genetic variation: caused by genes/alleles (e.g. Blood group). Environmental variation: caused by the environment (e.g. Scars). Both: many characteristics are influenced by both (e.g. Height: genetic potential plus nutrition). Continuous variation (range of values, e.g. Height) vs discontinuous variation (distinct categories, e.g. Blood group).
Priority 2: Natural selection
The four steps: 1) Variation exists in a population. 2) Competition for limited resources. 3) Individuals with advantageous characteristics are more likely to survive ("survival of the fittest"). 4) These individuals reproduce and pass on their advantageous alleles to the next generation.
Priority 3: Selective breeding
Humans choose parents with desired traits and breed them over many generations. Advantages: improved yields, disease resistance. Disadvantage: reduces genetic diversity, which makes populations vulnerable to new diseases.
Frequently asked questions
Is natural selection random?
The mutations that create variation are random, but the selection itself is not random: it favours organisms better adapted to their environment.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.