Cambridge IGCSE Biology 0610: Variation and selection – Study Guide

This guide teaches topic 18, Variation and selection, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. It covers sections 18.1 Variation, 18.2 Adaptive features and 18.3 Selection. Each section has Core statements, which every candidate needs, and Supplement statements, which are labelled Extended only here.

Quick links: course hub · printable checklist · revision notes · practice questions

What this topic covers

Section	What you must be able to do	Tier
18.1 (1–8)	Define variation; tell continuous from discontinuous variation and their causes; investigate examples; describe mutation and what increases its rate	Core
18.1 (9–10)	Describe gene mutation; list the sources of genetic variation	Extended only
18.2 (1–2)	Define an adaptive feature; describe adaptive features from images or information	Core
18.2 (3)	Explain the adaptive features of hydrophytes and xerophytes	Extended only
18.3 (1–3)	Describe natural selection and selective breeding; apply selective breeding to given contexts	Core
18.3 (4–6)	Describe adaptation as a process; antibiotic-resistant bacteria; natural vs artificial selection	Extended only

This topic builds directly on Inheritance: alleles, genotype and phenotype are used throughout.

18.1 Variation

Variation is the differences between individuals of the same species.

Continuous and discontinuous variation (Core)

	Continuous variation	Discontinuous variation
Phenotypes	A range between two extremes	A limited number, with no intermediates
Syllabus examples	Body length, body mass	ABO blood groups, seed shape in peas, seed colour in peas
Usual cause	Genes and the environment	Usually genes only
Graph	Histogram (bars touch)	Bar chart (bars separate)

Body mass depends on the alleles you inherit, but also on diet and exercise. Blood group depends only on your alleles: no diet changes group A into group B.

Investigating variation (Core)

The syllabus asks you to investigate examples. A typical investigation:

1. Choose a feature, for example hand span in a class, or leaf length on one tree.
2. Measure a large sample with the same method each time (same ruler, same starting point).
3. Record the data in a table, then group continuous data into equal class intervals.
4. Plot a histogram for continuous data, or a bar chart for discontinuous data.

Worked example 1. Thirty students measured their hand span.

Hand span / cm	16.0–17.9	18.0–19.9	20.0–21.9	22.0–23.9	24.0–25.9
Number of students	3	7	12	6	2

(a) Type of variation: continuous, because hand span can take any value in a range between two extremes.

(b) Percentage of students in the modal class: 12 ÷ 30 × 100 = 40%.

(c) Graph: a histogram, with hand span on the x-axis, number of students on the y-axis, and bars touching.

(d) Cause: both genes and the environment, for example nutrition during growth.

Mutation (Core)

• Mutation is genetic change.
• Mutation is the way in which new alleles are formed.
• Ionising radiation and some chemicals increase the rate of mutation.

Extended only

• A gene mutation is a random change in the base sequence of DNA. Link to 17.1: a changed base sequence can change the amino acid sequence, and so the shape of a protein.
• Sources of genetic variation in populations: mutation, meiosis, random mating and random fertilisation. Meiosis produces genetically different gametes. Random mating and random fertilisation mean any gamete can combine with any other, giving new combinations of alleles.

18.2 Adaptive features

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. The word “inherited” matters: a feature gained during life, such as a scar or larger muscles from training, is not an adaptive feature.

Interpreting information about a species (Core)

Exam questions often give you a picture or a description and ask you to describe the adaptive features. Use this pattern for every feature:

Feature → what it does → how that helps survival in this environment.

Worked example 2. A snow-dwelling hare has thick white fur in winter, short ears and large, furry hind feet.

• Thick fur traps a layer of air, which insulates the body and reduces heat loss in a cold climate.
• White fur gives camouflage against snow, so predators are less likely to see it.
• Short ears have a smaller surface area, so less heat is lost from them.
• Large, furry feet spread the animal’s weight so it does not sink into snow, helping it run from predators.

Extended only: hydrophytes and xerophytes

A hydrophyte is a plant adapted to living in water. A xerophyte is a plant adapted to dry conditions.

Hydrophyte feature (e.g. a water lily)	Explanation
Large air spaces in leaves and stems	Keep leaves floating near the surface for light; store air for gas exchange
Stomata on the upper surface of floating leaves	The lower surface is in water, so gas exchange happens with the air above
Thin or absent waxy cuticle	Water loss is not a problem
Little xylem and little support tissue	Water supports the plant; water is all around it
Large, flat leaves	Absorb as much light as possible at the surface

Xerophyte feature (e.g. cactus, marram grass)	Explanation
Thick waxy cuticle	Reduces evaporation from the leaf surface
Sunken stomata, hairs, rolled leaves	Trap moist air near the stomata, so the water vapour gradient is smaller and less water diffuses out
Fewer stomata; leaves reduced to spines	Smaller surface for water loss
Deep or widespread roots	Reach water deep underground or collect it quickly after rain
Water-storage tissue in a swollen stem	Keeps a supply of water for dry periods

In an answer, always link the feature to water (loss, uptake or storage) or, for hydrophytes, to light and gas exchange.

18.3 Selection

Natural selection (Core)

Learn the five steps from the syllabus in order:

1. There is genetic variation within the population.
2. Organisms produce many offspring, more than the environment can support.
3. There is a struggle for survival, including competition for resources such as food, water and mates, and pressure from predators and disease.
4. Individuals that are better adapted to the environment have a greater chance of reproducing.
5. These individuals pass on their alleles to the next generation.

Over many generations the alleles for the useful feature become more common in the population.

Extended only: adaptation and antibiotic resistance

Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Note the difference: an adaptive feature is a feature; adaptation is a process.

Development of antibiotic-resistant bacteria, step by step:

1. A random mutation gives one bacterium an allele for resistance to an antibiotic.
2. The antibiotic is used. Non-resistant bacteria are killed.
3. The resistant bacterium survives and has less competition.
4. It reproduces (asexually), passing the resistance allele to its offspring.
5. Over many generations a resistant strain forms. Bacteria reproduce quickly, so this happens fast.

Selective breeding (Core)

Selective breeding is artificial selection carried out by humans:

1. Humans select individuals with desirable features.
2. These individuals are crossed to produce the next generation.
3. Offspring showing the desirable features are selected and bred.
4. This is repeated over many generations.

It is used to improve crop plants (higher yield, disease resistance, drought tolerance) and domesticated animals (more milk, more meat, calmer temperament).

Worked example 3. A breeder wants a wheat variety with higher yield. Average yields over four rounds of selection were 3.2, 3.5, 3.9 and 4.3 tonnes per hectare.

• Method: grow many plants, choose those with the highest grain yield, cross them, grow their seeds, choose the highest-yielding offspring again, and repeat.
• Percentage increase: (4.3 − 3.2) ÷ 3.2 × 100 = 34.4%.
• It takes many generations because only some offspring inherit all the desired alleles each time.

Extended only: natural vs artificial selection

Natural selection	Artificial selection
The environment decides which individuals survive and reproduce	Humans decide which individuals reproduce
Features selected help survival and reproduction	Features selected are useful or desirable to humans
Usually slower	Usually faster
Leads to populations suited to their environment	Leads to varieties and breeds suited to human needs

Common errors

• Giving height or mass as discontinuous because people are measured in whole numbers. The feature is continuous.
• Saying discontinuous variation is caused by “the environment only”.
• Writing “mutation is a change in DNA caused by radiation”. Mutation is genetic change; radiation only increases the rate.
• Describing an adaptive feature without saying how it helps survival or reproduction.
• Writing “the bacteria became resistant because of the antibiotic”. The mutation was already there; the antibiotic selected it.
• Saying individuals “adapt” or “evolve” during their lifetime. Populations change over generations.
• Missing “over many generations” in selective breeding answers.

Next steps

• Condensed recall and a self-test: Variation and selection revision notes
• Exam-style questions with answers: Variation and selection practice questions
• Find your gaps: Core diagnostic or Extended diagnostic
• Tick off every statement on the 0610 checklist and return to the course hub

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.