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Detailed notes on Variation and Selection for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Why no two individuals are quite alike. Cambridge wants the categories of variation and where they come from.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Smooth range vs distinct categories.
Continuous variation.
Discontinuous variation.
Examples to memorise:
| Trait | Type |
|---|---|
| Height | Continuous |
| Weight | Continuous |
| Hand span | Continuous |
| Reaction time | Continuous |
| Blood group | Discontinuous |
| Tongue rolling | Discontinuous |
| Pea seed colour | Discontinuous |
| Earlobe attachment | Discontinuous |
Worked qualitative. Why is blood group discontinuous but height continuous?
Cambridge tip. Always pair the type of variation with the type of GRAPH (smooth curve vs bar chart). Cambridge often gives data and asks for the graph.
Genes (meiosis + fertilisation + mutation) + environment.
1. Genetic variation.
Sources:
Without these, all individuals would be identical clones (asexual reproduction).
2. Environmental variation.
Same genome, different conditions → different outcomes.
Examples:
3. Most traits = both.
Worked qualitative. Identical twins separated at birth often grow up to similar heights but different weights. Why?
Cambridge tip. When asked about CAUSE of variation, give BOTH genetic and environmental sources. Just one is partial.
Random DNA changes. Mostly neutral or harmful, occasionally beneficial. Drive evolution.
Definition. A mutation is a random change in the DNA sequence (or chromosome structure).
Where mutations come from:
Types of mutation:
Effects:
Why mutations matter:
Examples of beneficial mutations:
Worked qualitative. Why is exposure to UV radiation linked to skin cancer?
Cambridge tip. Always say 'random change in DNA' — the word RANDOM matters. Mutations are not directed by environment; the environment just SELECTS which random mutations spread.
Verbatim phrases and definitions Cambridge mark schemes credit.
Variation appears Paper 4 (5-7 marks: classify trait, identify cause, plot data). Examiner reports flag students confusing variation type with cause type.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (18.1); 0610/42 Oct/Nov 2024 — Q19 (variation); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on variation , written exactly the way a tutor would explain them at the board.
Question
Distinguish between continuous and discontinuous variation, with an example of each.
Step-by-step solution
Step 1
Continuous variation is variation with a range of values between two extremes, with no distinct groups (it is measured).
Step 2
Example: human height or mass — there is a smooth range. Shown on a histogram (often a bell-shaped curve).
Step 3
Discontinuous variation is variation with distinct categories and no in-between values (it is counted).
Step 4
Example: blood group (A, B, AB, O) or tongue-rolling (can or cannot). Shown on a bar chart.
Answer
Continuous: a smooth range (e.g. height) — histogram. Discontinuous: distinct categories (e.g. blood group) — bar chart.
Question
State whether each shows continuous or discontinuous variation: (a) the mass of apples; (b) human blood group; (c) the length of leaves; (d) whether a person has free or attached ear lobes.
Step-by-step solution
Step 1
(a) Mass of apples → a smooth range → continuous.
Step 2
(b) Blood group → distinct categories → discontinuous.
Step 3
(c) Length of leaves → a smooth range → continuous.
Step 4
(d) Free or attached ear lobes → two categories → discontinuous.
Answer
(a) Continuous; (b) discontinuous; (c) continuous; (d) discontinuous.
Examiner tip
Measured features with a range = continuous; features in clear categories = discontinuous.
Question
Explain how both genes and the environment can cause variation, giving an example of each.
Step-by-step solution
Step 1
Genetic variation is caused by differences in genes (alleles), which arise through meiosis, random fertilisation and mutation.
Step 2
Example: blood group is determined only by genes.
Step 3
Environmental variation is caused by differences in the surroundings, such as diet, exercise or sunlight.
Step 4
Example: a person's skin tanning in the sun, or scars from injury. Many features (e.g. height) are affected by both genes and environment.
Answer
Genes cause inherited variation (e.g. blood group). The environment causes variation through conditions like diet and sunlight (e.g. tanning). Many features result from both.
Question
Define a mutation and explain why mutations are important for variation.
Step-by-step solution
Step 1
A mutation is a random change in the base sequence of DNA (the genetic material).
Step 2
A mutation can create a new allele of a gene.
Step 3
New alleles are the original source of all genetic variation in a species.
Step 4
This variation is important because it provides the differences that natural selection acts on, allowing populations to adapt.
Answer
A mutation is a random change in the DNA base sequence, which can produce a new allele. Mutations are the original source of genetic variation, on which natural selection acts.
Question
Student A measures the heights of 100 people and gets a smooth range of values; student B records the blood groups of 100 people and gets four distinct numbers. Explain what type of variation each is showing and the correct way to display each.
Step-by-step solution
Step 1
Height gives a smooth range of values with no gaps, so it is continuous variation.
Step 2
Continuous data is best displayed on a histogram, which usually shows a bell-shaped curve.
Step 3
Blood group gives a few distinct categories (A, B, AB, O) with no in-between values, so it is discontinuous variation.
Step 4
Discontinuous data is best displayed on a bar chart, with a separate bar for each category.
Answer
Height = continuous (smooth range) → histogram. Blood group = discontinuous (distinct categories) → bar chart.
Examiner tip
Continuous → histogram (curve); discontinuous → bar chart (separate bars). Choosing the right graph is commonly tested.
Question
Explain why mutation is essential for evolution, and name two factors that increase the rate of mutation.
Step-by-step solution
Step 1
Mutations are the only way new alleles are made.
Step 2
Without new alleles, there would be no new variation, so natural selection would have nothing new to act on and the species could not evolve new features.
Step 3
Most mutations are neutral or harmful, but occasionally one is beneficial, and this can spread through the population by natural selection.
Step 4
The mutation rate is increased by ionising radiation (e.g. X-rays, gamma rays, ultraviolet light) and by some chemicals (e.g. those in tobacco smoke).
Answer
Mutation is the only source of new alleles, so without it there would be no new variation to evolve from. Its rate is increased by ionising radiation and certain chemicals.
Examiner tip
New alleles only come from mutation, so it is the ultimate source of variation. Radiation and certain chemicals raise the mutation rate.
High-scoring sample answers for variation on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by variation. (1 mark)
Model answer
Variation is the differences between individuals of the same species.
Why this scores
One mark for 'differences between individuals of the same species'.
Give one example of a feature that shows continuous variation and one that shows discontinuous variation in humans. (2 marks)
Model answer
Continuous: height (or mass). Discontinuous: blood group (or tongue-rolling / ear lobe type).
Why this scores
One mark each: a valid continuous feature; a valid discontinuous feature.
Explain the difference between continuous and discontinuous variation. (3 marks)
Model answer
Continuous variation has a range of values between two extremes, with no distinct groups (for example, height). Discontinuous variation has a number of distinct categories, with no intermediate values (for example, blood group). Continuous variation is usually shown on a histogram and discontinuous variation on a bar chart.
Why this scores
Three marks: continuous = a range/no distinct groups; discontinuous = distinct categories/no in-between; correct graph types or examples.
Explain how genes and the environment can each cause variation between individuals. (4 marks)
Model answer
Genetic variation is caused by differences in the alleles that individuals inherit; these differences arise through meiosis, random fertilisation and mutation. For example, a person's blood group is determined only by their genes. Environmental variation is caused by differences in the conditions an individual experiences, such as diet, exercise or exposure to sunlight; for example, a person's skin may become darker (tanned) in the sun. Many features, such as height, are affected by both the genes and the environment.
Why this scores
Four marks: genetic variation from differences in alleles (meiosis/fertilisation/mutation); a genetic example; environmental variation from conditions (diet/sun); an environmental example.
Explain what is meant by a mutation and why mutations are important as a source of variation. (5 marks)
Model answer
A mutation is a random change in the base sequence of DNA. Such a change can produce a new allele of a gene. Because mutation is the only way that new alleles are formed, it is the original source of all genetic variation in a species. This variation is important because it provides the differences between individuals that natural selection can act on. Occasionally a mutation produces a beneficial allele, which can then spread through the population over generations, allowing the species to adapt to its environment.
Why this scores
Five marks: mutation = random change in DNA; produces new alleles; the source of genetic variation; provides differences for natural selection; occasionally beneficial → spreads/adaptation.
Describe the causes of variation in a population, and explain why mutation is described as the ultimate source of all variation. (6 marks)
Model answer
Variation between individuals of a species is caused by their genes (alleles) and by the environment. Genetic variation arises because individuals inherit different combinations of alleles; this comes about through the random separation of chromosomes in meiosis, the random fusion of gametes at fertilisation, and mutation. The environment also causes variation, through factors such as diet, exercise and sunlight, and many features result from genes and environment together. However, meiosis and fertilisation only shuffle alleles that already exist — they do not make any new ones. Only a mutation, a random change in the DNA base sequence, can create a completely new allele. For this reason mutation is the ultimate (original) source of all variation: without mutations there would be no new alleles to be shuffled by meiosis and fertilisation, so there would be no new genetic variation for natural selection to act on, and the species could not evolve.
Why this scores
Up to 6 marks: variation caused by genes and environment; genetic variation from meiosis/fertilisation/mutation; environment (diet/sun); meiosis and fertilisation only shuffle existing alleles; only mutation makes new alleles; so mutation is the ultimate source.
Definitions to memorise and the exact keywords mark schemes credit for variation answers — sharpened from recent examiner reports for the 2026 0610 sitting.
The differences between individuals of the same species.
Variation showing a range of values between two extremes, with no distinct groups (e.g. height). Shown on a histogram.
Variation showing distinct categories with no intermediates (e.g. blood group). Shown on a bar chart.
A random change in the base sequence of DNA. It can produce a new allele and is the source of all genetic variation.
The traps other students keep falling into on variation questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Confusing two different classifications.
How to avoid it
Continuous/discontinuous describes the PATTERN of variation; genes/environment describes the CAUSE. They are independent — discontinuous variation (e.g. blood group) is usually purely genetic.
Why it happens
We hear most about disease-causing mutations.
How to avoid it
Most mutations are neutral or harmful, but a few are beneficial — and these can be selected for, driving evolution.
Why it happens
The two graphs look similar.
How to avoid it
Continuous data → histogram (bars touch). Discontinuous categories → bar chart (separate bars).
The things students keep getting wrong in this sub-topic, answered.