Cambridge IGCSE Biology 0610. Topic 18: Variation and selection
Variation & Selection
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Continuous variation shows a full range (e.g. Height, many genes plus environment); discontinuous variation falls into distinct categories (e.g. Blood group, one or few genes). Natural selection is a fixed five-step chain: variation from mutation, competition, survival of the best-adapted, reproduction passing on the allele, and rising allele frequency over generations.
How Cambridge tests this topic
This topic lives or dies on one six-mark answer, natural selection, which has a fixed five-link chain that examiners mark like a checklist. Around it sit reliable classification marks (continuous vs discontinuous) and the natural-vs-selective-breeding comparison. Memorise the selection chain word for word, learn what causes genetic variation, and you can score nearly full marks every cycle.
Focused lessons
Break the topic into manageable skills
Use these guides when one concept, calculation or question pattern needs more attention than the full-topic overview.
Continuous variation shows a full range of values between two extremes (e.g. Height, mass), is controlled by many genes and is influenced by the environment. It gives a smooth bell-shaped distribution. Discontinuous variation falls into distinct categories with no in-between (e.g. Blood group A/B/AB/O, tongue rolling), is controlled by one or a few genes and is unaffected by the environment. It gives a bar-chart distribution. To classify an unknown feature, ask: full range or separate boxes? Causes of genetic variation are a standing list: mutation (the source of new alleles), meiosis, and random fertilisation. The environment causes variation too but is not inherited (e.g. A scar, a sun tan).
Mutation: define it precisely
A mutation is a random change in the base sequence of DNA / in a gene. It is the original source of all new alleles and therefore of genetic variation. Mutation rate is increased by named mutagens: ionising radiation (e.g. X-rays, gamma rays, UV) and certain chemicals (e.g. Those in tobacco tar). Most mutations are neutral or harmful, but occasionally a mutation produces a useful new allele, which is exactly what natural selection then acts on. This sentence is the bridge between the two halves of the topic and is worth stating explicitly in long answers.
Natural selection: the five-link chain
The flagship answer. Write all five links and you bank the marks:
Within a population there is genetic variation, caused by mutation.
Organisms produce more offspring than the environment can support, so there is competition for resources (a struggle for survival).
Individuals with variations best adapted to the environment are more likely to survive (survival of the fittest).
The survivors reproduce and pass on their advantageous alleles to their offspring.
Over many generations the frequency of the advantageous allele increases in the population.
The two contexts the exam reuses: antibiotic-resistant bacteria and a named adaptation (e.g. Peppered moths, beak shape). Always keep the causation correct. The environment selects existing variation; it does not create the adaptation on demand.
Natural vs selective breeding (artificial selection)
Selective breeding is artificial selection: humans choose which organisms with desired features to breed together, repeating over many generations so the desired feature becomes more common. The contrast with natural selection is the selecting agent, humans choose in selective breeding, the environment selects in natural selection. But the underlying mechanism (selecting from existing variation and breeding the chosen) is the same. Examples examiners use: crops with higher yield, cattle with more milk, disease-resistant varieties. A common question asks you to outline the selective-breeding process: select parents with the desired feature → breed them → select the best offspring → repeat over many generations. Naming 'over many generations' is a standing mark in both natural and artificial selection answers.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
Adaptive feature: An inherited feature that helps an organism to survive and reproduce in its environment.
Continuous variation: Variation that shows a complete range of phenotypes between two extremes, with no distinct categories. Examples include height, mass and foot length. Continuous variation is typically controlled by many genes (polygenic) and influenced by the environment.
Discontinuous variation: Variation that falls into distinct, separate categories with no intermediates. Examples include blood group (A, B, AB, O), tongue rolling (can or cannot) and eye colour. Discontinuous variation is usually controlled by a single gene and is not influenced by the environment.
Evolution: The gradual change in the inherited characteristics of a population over many generations, which may result in the formation of new species. Evolution occurs through natural selection, where organisms with favourable characteristics are more likely to survive and reproduce.
Mutation: A random genetic change, which may produce a new allele or change a gene or chromosome.
Natural selection: The process by which individuals with variation better suited to the environment are more likely to survive and reproduce, passing on their alleles, so the favourable feature becomes more common over generations.
Selective breeding: The process by which humans choose organisms with desirable characteristics and breed them together over many generations to enhance those traits. Also called artificial selection. Examples include breeding cows for high milk yield or wheat for disease resistance.
Variation: Differences between individuals of the same species. Continuous variation (e.g. Height) shows a complete range and is affected by many genes and environment; discontinuous variation (e.g. Blood group) has distinct categories and is usually controlled by genes alone.
Process diagramThe natural selection sequenceThe five-link chain examiners mark like a checklist. Write every step for full marks.
1
Genetic variation exists in the population (caused by mutation)
2
Over-production of offspring → competition for resources
3
Best-adapted individuals are more likely to survive
4
Survivors reproduce and pass on the advantageous allele
5
Allele frequency increases over many generations
Try a worked question (5 marks)
A species of beetle lives on tree bark. Most beetles are brown, but a few are green. The trees they live on have brown bark. Explain how natural selection has resulted in most beetles being brown.
Show the answer and marking guidance
there is variation in colour (caused by mutation); brown beetles are better camouflaged on brown bark; so they are less likely to be eaten by predators / more likely to survive; survivors reproduce and pass on the allele for brown colour to offspring; over many generations the proportion of brown beetles increases;
Each semicolon separates a distinct answer point. This is original practice content, not an official Cambridge past-paper question.
Common mix-ups to avoid
Use these as a correction checklist after attempting a question.
Saying the environment 'creates' an adaptation. It selects from variation that already exists.
Classifying variation wrongly: continuous = full range (many genes + environment); discontinuous = distinct categories (one/few genes).
Omitting 'over many generations' from a selection answer. It is a standing mark.
Defining mutation vaguely. It is a change in the base sequence of DNA / a gene.
Forgetting that mutation is the source of new alleles that natural selection then acts on.
Confusing selective breeding and natural selection. The difference is the selecting agent (humans vs environment).
How to build a complete longer answer
Natural-selection six-markers are the most predictable extended answer in the whole syllabus. Answer with the five-link chain every time: variation (from mutation) → competition/over-production → best-adapted survive → reproduce and pass on alleles → frequency increases over generations. Slot the question's specific context into each link (camouflage, antibiotic, drought tolerance). For selective breeding, mirror the chain but make humans the selector and add 'repeat over many generations'. The single most common reason students miss marks is dropping the final 'over many generations the allele frequency increases' line. Write it explicitly.
Selective breeding lands in a familiar Malaysian context through oil palm and rubber improvement and high-yield padi varieties. Past papers have used agricultural examples local students recognise. The natural-selection chain is identical to the antibiotic-resistance answer in the drugs topic, so revise the two together: learning one gives you the other for free.
Check your understanding
Practice this topic without notes
Attempt the questions before opening the marking guidance.
What is the difference between continuous and discontinuous variation?
Continuous variation shows a complete range of values between two extremes (e.g. Height), controlled by many genes and the environment. Discontinuous variation falls into distinct categories with no in-between (e.g. Blood group), usually controlled by one or a few genes.
How does natural selection work?
Variation exists from mutation; organisms over-produce so there is competition; the best-adapted individuals are more likely to survive; they reproduce and pass on the advantageous allele; over many generations its frequency increases. The environment selects existing variation. It does not create it.
What is a mutation?
A mutation is a random change in the base sequence of DNA. It is the original source of new alleles and therefore of genetic variation. Mutation rate is increased by mutagens such as ionising radiation and some chemicals.