Cambridge IGCSE Biology 0610. Topic 18: Variation and selection

Variation & Selection

By the IGCSE Biology Team · reviewed for accuracy by our Biology specialists · last reviewed June 2026 · about us

In short

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.

The exam angle

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.

Types of variation and their causes

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.

Process diagram The natural selection sequence The five-link chain examiners mark like a checklist. Write every step for full marks.

Genetic variation exists in the population (caused by mutation)
Over-production of offspring → competition for resources
Best-adapted individuals are more likely to survive
Survivors reproduce and pass on the advantageous allele
Allele frequency increases over many generations

Worked exam 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 mark-scheme answer

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 point separated by a semicolon is worth one mark. This is how real Cambridge mark schemes are written.

Common mistakes that cost marks

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 phrase the 6-mark answer on this topic

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.

Learn the full 6-mark technique →

For students in Malaysia

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.

Frequently asked questions

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.

Exam guides for this topic

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Variation & Selection

Types of variation and their causes

Mutation: define it precisely

Natural selection: the five-link chain

Natural vs selective breeding (artificial selection)

Worked exam question (5 marks)

Common mistakes that cost marks

How to phrase the 6-mark answer on this topic

For students in Malaysia

Frequently asked questions

Frequently asked questions

Related topics

Exam guides for this topic