This guide covers IGCSE Biology topic 18: variation and selection. You will revise continuous and discontinuous variation, natural selection (mutations, competition, survival, reproduction), artificial selection (selective breeding), and evolution.

Key concepts to revise

• Variation – Continuous (e.g. height, mass; range of values) vs discontinuous (e.g. blood group; distinct categories). Caused by genes, environment, or both.
• Natural selection – Variation exists; competition for resources; better-adapted individuals survive and reproduce; alleles passed on; population changes over time.
• Artificial selection – Humans choose organisms with desired traits to breed; repeated over generations; e.g. crop yield, milk production.
• Evolution – Change in species over time; natural selection is the mechanism; evidence: fossils, comparative anatomy.

Exam tips and command words

• State – One example of continuous and one of discontinuous variation.
• Describe – The process of natural selection (variation → competition → survival → reproduction → allele frequency changes).
• Explain – Why antibiotic resistance is an example of natural selection (mutations; resistant bacteria survive; pass on resistance).
• Compare – Natural vs artificial selection (who selects; what is selected for).

Common mistakes

• Saying “organisms adapt” in order to survive (variation exists first; selection acts on it; individuals do not “choose” to adapt).
• Confusing natural selection (environment; survival of fittest) with artificial selection (humans choose).
• Not stating that mutation is the source of new variation (random; may be beneficial, harmful or neutral).

Revision checklist

• Give one example each of continuous and discontinuous variation and state causes (genes, environment).
• Describe natural selection in four steps (variation, competition, survival, reproduction).
• Explain antibiotic resistance or another example of natural selection.
• Describe artificial selection (selective breeding) with one example.

Next steps

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