Cambridge IGCSE Biology 0610: Variation and selection – Revision Notes

For full explanations and worked examples, use the Variation and selection study guide. These notes are for the final weeks.

They cover topic 18 of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028: 18.1 Variation, 18.2 Adaptive features and 18.3 Selection. Every section has Core statements for all candidates; the Supplement statements are labelled Extended only.

Links: course hub · checklist · practice questions · Inheritance revision notes · Core diagnostic · Extended diagnostic

Definitions to learn word for word

Term	Definition	Tier
Variation	Differences between individuals of the same species	Core
Continuous variation	Results in a range of phenotypes between two extremes	Core
Discontinuous variation	Results in a limited number of phenotypes with no intermediates	Core
Mutation	Genetic change	Core
Adaptive feature	An inherited feature that helps an organism to survive and reproduce in its environment	Core
Gene mutation	A random change in the base sequence of DNA	Extended only
Adaptation	The process, resulting from natural selection, by which populations become more suited to their environment over many generations	Extended only

18.1 Variation

Continuous vs discontinuous (Core)

	Continuous	Discontinuous
Phenotypes	Range between two extremes	Limited number, no intermediates
Examples in the syllabus	Body length, body mass	ABO blood groups, seed shape in peas, seed colour in peas
Caused by	Genes and environment	Usually genes only
Data display	Histogram, bars touching, equal class intervals	Bar chart, bars separate
Data type	Measured	Counted into categories

Method: deciding the type of variation

1. Can the feature take any value in a range (length, mass, time)? → continuous.
2. Does every individual fall into one of a few clear categories? → discontinuous.
3. Check for intermediates: if there are none, it is discontinuous.

Mutation (Core)

• Mutation = genetic change.
• It is how new alleles form.
• Rate increased by ionising radiation (for example X-rays, gamma rays) and some chemicals.

Extended only

• Gene mutation = random change in the base sequence of DNA.
• Four sources of genetic variation in populations: mutation, meiosis, random mating, random fertilisation.

18.2 Adaptive features

• Core: define, then interpret images or information about a species. For every feature say what it does and how that helps survival or reproduction in that environment.
• The feature must be inherited. Something gained during life does not count.

Extended only: hydrophytes and xerophytes

Plant type	Features	Links to
Hydrophyte	Large air spaces; stomata on upper surface of floating leaves; thin or no cuticle; little xylem and support tissue; large flat leaves	Buoyancy, light, gas exchange with air
Xerophyte	Thick waxy cuticle; sunken stomata; hairs; rolled leaves; few stomata; spines instead of leaves; deep or wide roots; water-storage tissue	Less water loss, more water uptake, water storage

Why sunken stomata, hairs and rolled leaves work: they trap moist air next to the stomata, so the water vapour concentration gradient is smaller and less water vapour diffuses out.

18.3 Selection

Method: any natural selection answer (Core)

1. Variation: some individuals have a different allele or feature.
2. Many offspring are produced.
3. Struggle for survival / competition for named resources (or predation, disease).
4. Better-adapted individuals survive and reproduce more.
5. They pass on their alleles.
6. Over many generations the allele becomes more common.

Method: antibiotic resistance (Extended only) Random mutation → resistance allele in a few bacteria → antibiotic kills non-resistant bacteria → resistant ones survive, less competition → reproduce and pass on allele → resistant strain over many generations.

Method: selective breeding (Core) Humans select individuals with desirable features → cross them → select offspring showing the desirable features → breed from them → repeat over many generations.

Examples of desirable features: crop plants with higher yield, disease resistance or drought tolerance; animals with higher milk yield, more meat or calmer temperament. In an “apply to a given context” question, name the exact feature from the context.

Small worked reminders

Applying natural selection to a context (Core). A population of lizards lives on rocks. Some have dark skin and some pale skin, because of different alleles. Birds eat lizards they can see easily. After a volcanic eruption the rocks are covered with dark ash.

• Variation: dark and pale lizards exist.
• Struggle for survival: birds see pale lizards on dark ash more easily, so more pale lizards are eaten.
• Better adapted: dark lizards survive and reproduce more.
• Alleles: dark lizards pass on the allele for dark skin.
• Over many generations the proportion of dark lizards increases.

Notice that no lizard changes colour. The proportion in the population changes.

Applying selective breeding to a context (Core). A farmer wants chickens that lay more eggs.

• Select the hens that lay the most eggs, and cockerels from hens that laid the most eggs.
• Cross them.
• From the offspring, select the hens that lay the most eggs and breed from them.
• Repeat for many generations.

Always name the specific feature from the question (here, number of eggs laid) instead of writing “the best feature”.

Reading a variation table. If a table gives the number of individuals in each height class, the class with the highest number is the modal class. To find a percentage, divide that number by the total and multiply by 100. Check the total first: questions often give totals that are easy to misread.

Investigating variation (Core). Measure a large sample, use the same method every time, group continuous data into equal class intervals, then choose the right graph. A larger sample gives a more reliable picture of the variation in the population.

Must-know distinctions

• Continuous vs discontinuous: a range vs a few categories. Genes and environment vs usually genes only.
• Adaptive feature vs adaptation (Extended): a feature vs a process over generations.
• Mutation vs gene mutation: Core “genetic change” vs Extended “random change in the base sequence of DNA”.
• Mutation vs cause of mutation: radiation and chemicals increase the rate; mutation also happens without them.
• Natural vs artificial selection (Extended): the environment chooses vs humans choose; features that aid survival vs features humans want; usually slower vs usually faster.
• Individual vs population: individuals do not adapt during their lifetime; populations change over generations.

Quick self-test

1. Define variation.
2. Classify each as continuous or discontinuous: (a) body mass, (b) seed colour in peas, (c) leaf length, (d) ABO blood group.
3. State which type of variation is usually caused by genes only.
4. Name the type of graph used to show continuous variation and describe how its bars are drawn.
5. Pea plants that are heterozygous for seed shape are crossed and produce 800 seeds. Round is dominant to wrinkled. How many wrinkled seeds are expected, and what type of variation does seed shape show?
6. State two factors that increase the rate of mutation.
7. State four sources of genetic variation in populations. (Extended)
8. A camel has broad, flat feet. Suggest how this is an adaptive feature for living in a sandy desert.
9. Explain how rolled leaves reduce water loss in a xerophyte. (Extended)
10. State what humans select in the first step of selective breeding.
11. Explain why a resistant strain of bacteria develops faster than a new variety of cattle. (Extended)
12. State two differences between natural selection and artificial selection. (Extended)

Answers

1. Differences between individuals of the same species.
2. (a) Continuous; (b) discontinuous; (c) continuous; (d) discontinuous.
3. Discontinuous variation.
4. A histogram; the bars touch and the class intervals are equal.
5. 3 round : 1 wrinkled, so one quarter of 800 = 200 wrinkled. Seed shape shows discontinuous variation.
6. Ionising radiation and some chemicals.
7. Mutation, meiosis, random mating, random fertilisation.
8. Broad feet spread the camel’s weight, so it does not sink into sand and can walk to find food and water, helping it survive.
9. Rolling traps moist air inside the roll next to the stomata; the water vapour gradient is smaller, so less water diffuses out.
10. Individuals with desirable features.
11. Bacteria reproduce much faster, so many more generations pass in the same time, and each generation lets the resistance allele become more common.
12. Any two: the environment selects vs humans select; features selected aid survival vs features useful to humans; natural selection is usually slower.

Where marks are usually lost

• Calling body length discontinuous because values are recorded to the nearest centimetre.
• Drawing a bar chart with gaps for continuous data, or a histogram for blood groups.
• Stating that continuous variation is caused by “the environment”, without genes.
• Writing that radiation “causes mutation” as a definition. Mutation is genetic change; radiation increases the rate.
• Listing adaptive features without explaining how each one helps survival or reproduction.
• For xerophytes, writing “stops water loss”. Features reduce water loss.
• In natural selection answers, leaving out passing on alleles or over many generations.
• Saying bacteria “become immune” or “mutate because of the antibiotic”. The mutation is random; the antibiotic selects.
• In selective breeding, forgetting to select the offspring and repeat.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge University Press & Assessment. Topic 18 Variation and selection, sections 18.1–18.3.