IGCSE Biology: Variation and Selection Practice Questions
Variation is the differences between individuals of the same species. Continuous variation gives a range of values with no distinct categories, while discontinuous variation gives a limited number of distinct phenotypes with no intermediates.
Topic 18 of Cambridge IGCSE Biology 0610 contains the single most valuable answer in the whole syllabus: the four step account of natural selection. It reappears in the drugs topic, the ecology topic and in data questions. The questions below drill that account and the contrast with selective breeding.
What you need to know for Variation and Selection
- Continuous variationVariation with a range of values between two extremes and no distinct categories, such as height or mass. It is usually caused by several genes together with the environment, and is plotted as a histogram or curve.
- Discontinuous variationVariation with a limited number of distinct phenotypes and no intermediates, such as human blood group. It is usually controlled by one or a few genes and is unaffected by the environment. It is plotted as a bar chart.
- MutationA genetic change. Gene mutation is a change in the base sequence of DNA. Mutation is the way new alleles arise, and its rate is increased by ionising radiation and by some chemicals.
- Adaptive featureAn inherited feature that helps an organism survive and reproduce in its environment.
- Natural selectionVariation exists within a population. Organisms with features better suited to the environment are more likely to survive and reproduce. They pass on the alleles for those features, so the proportion of individuals with them increases over generations.
- Selective breedingHumans select individuals with desired features, breed them together, select the best offspring and repeat over many generations. The selection pressure comes from humans rather than the environment.
IGCSE Biology Variation and Selection questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain how natural selection can make a population of insects resistant to an insecticide.
Show the worked answer
- There is variation within the insect population, caused by random mutation.
- By chance some insects carry an allele that makes them resistant to the insecticide. This allele exists before the insecticide is used.
- When the insecticide is applied, non-resistant insects are killed. Resistant insects survive.
- The survivors reproduce and pass the resistance allele on to their offspring.
- Over successive generations the proportion of resistant insects in the population increases, until the insecticide is no longer effective.
Explain the difference between continuous and discontinuous variation, giving one human example of each.
Show the worked answer
- Continuous variation produces a range of values between two extremes with no distinct groups, and individuals can take any value in that range.
- An example in humans is height, or body mass. It is influenced by many genes and by the environment, such as diet.
- Discontinuous variation produces a limited number of distinct categories with no intermediates.
- An example in humans is blood group, A, B, AB or O. It is controlled by a single gene and is not affected by the environment.
Describe how a farmer could use selective breeding to increase the milk yield of a herd of cattle, and state one difference between selective breeding and natural selection.
Show the worked answer
- Identify the cows in the herd with the highest milk yield, and a bull whose mother and daughters have high yields.
- Breed these selected individuals together.
- Measure the milk yield of the offspring and select the highest yielding animals from that generation.
- Repeat the process over many generations. The mean milk yield of the herd increases each generation.
- The difference from natural selection is that the selection pressure is applied by humans choosing which individuals breed, rather than by the environment determining which survive.
State what is meant by a gene mutation and give one factor that increases the rate at which mutations occur.
Show the worked answer
- A gene mutation is a change in the base sequence of the DNA of a gene.
- This can change the sequence of amino acids in the protein the gene codes for, which may change the protein's shape and function.
- Mutation happens naturally at a low rate and is the source of all new alleles.
- The rate is increased by ionising radiation, such as X rays, gamma rays and ultraviolet light, and by certain chemicals, for example those in tobacco tar.
Common mistakes in this topic
- Writing that organisms adapt during their lifetime in response to the environment.
- Saying mutations happen because they are needed. Mutations are random.
- Confusing selective breeding with genetic modification. Selective breeding uses normal reproduction over generations.
- Plotting continuous variation as a bar chart. Continuous data needs a histogram or line graph.
- Forgetting to mention that the allele is passed to offspring. Without inheritance there is no evolution.
Exam tips
- Learn natural selection as five marks: variation, mutation as its source, differential survival, reproduction, inheritance over generations.
- Use the phrase more likely to survive rather than survive. Selection is probabilistic.
- For any resistance question, insects, bacteria or weeds, use the same five step structure.
- Say the proportion of the population increases rather than the individuals change.
- Remember one disadvantage of selective breeding: reduced genetic variation, leaving the population vulnerable to disease.
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Variation and Selection FAQs
What is the difference between continuous and discontinuous variation?
Continuous variation shows a range of values between two extremes with no distinct categories, such as height, and is usually controlled by several genes and influenced by the environment. Discontinuous variation shows a limited number of distinct categories with no intermediates, such as blood group, and is usually controlled by one gene.
How does natural selection work?
Variation exists in a population, arising from random mutation. Individuals with features better suited to their environment are more likely to survive and reproduce. They pass the alleles for those features to their offspring, so over successive generations the proportion of the population carrying those alleles increases.
What is the difference between natural selection and selective breeding?
In natural selection the environment determines which individuals survive long enough to reproduce. In selective breeding humans choose which individuals reproduce, based on characteristics they find useful. Both work by allowing only some individuals to pass on their alleles, but the selection pressure comes from different sources.
Are mutations always harmful?
No. Most mutations have no effect, some are harmful, and a small number are beneficial. Beneficial mutations produce new alleles that may improve survival in a particular environment, and these are the raw material on which natural selection acts.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.