Variation and evolution — AQA GCSE Combined Science: Trilogy
Causes of variation, mutations, evolution by natural selection, selective breeding and genetic engineering.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Variation is the differences in the characteristics of individuals in a population. It may be due to the genes they have inherited, the conditions in which they have developed, or a combination of both. All variants arise from mutations: most have no effect on the phenotype, some influence the phenotype and very few determine the phenotype.
Evolution is a change in the inherited characteristics of a population over time through a process of natural selection, which may result in the formation of a new species. Individuals best suited to their environment are more likely to survive, breed and pass on their alleles. Populations that can no longer interbreed to produce fertile offspring are separate species.
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Parents with the desired characteristic are chosen and bred together, then the best offspring are bred again over many generations. It gives higher yields and disease-resistant crops, but it can lead to inbreeding and reduces the number of different alleles.
Genetic engineering is a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic. Bacteria have been engineered to produce human insulin to treat diabetes. GM crops can be resistant to insects, herbicides or disease and generally show increased yields, but some people have concerns about their effects.
Exam questions
5 exam-style questions (16 marks), each with its mark scheme.
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The whole of inheritance, variation and evolution on one page, so you can see where this subtopic fits.
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Quick check questions
Give the three possible causes of variation.
Show answer
Genetic causes, environmental causes, or a combination of genes and the environment.
What do all variants arise from?
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Mutations.
When have two populations become two new species?
Show answer
When they are so different in phenotype that they can no longer interbreed to produce fertile offspring.
Name one problem caused by selective breeding.
Show answer
Inbreeding, where some breeds are prone to disease or inherited defects.
What is a GM crop?
Show answer
A crop that has had its genes modified by genetic engineering.
Frequently asked questions
How does natural selection lead to evolution?
Natural selection leads to evolution because individuals in a population vary, and those with phenotypes best suited to their environment are more likely to survive and breed. They pass on the alleles for the useful characteristic to their offspring. Over many generations, a greater proportion of the population has the characteristic, so the inherited characteristics of the population change.
What are the 4 stages of natural selection?
The 4 stages of natural selection are: variation, as individuals have different phenotypes caused by their genes; pressure from the environment, such as predators, disease or competition; survival and breeding of the best-suited individuals; and passing on their alleles, so the characteristic becomes more common over many generations. Populations evolve, not individuals.
What is selective breeding in plants?
Selective breeding in plants is when humans choose parent plants with a desired characteristic, such as disease resistance or large flowers, and breed them together. The offspring with the characteristic are then bred again, over many generations, until all the offspring show it. A risk is that it reduces the number of different alleles in the population.
What is genetic engineering in agriculture?
Genetic engineering in agriculture means modifying the genome of a crop by introducing a gene from another organism to give a desired characteristic. These genetically modified (GM) crops can be resistant to insect attack, herbicides or disease, or produce bigger, better fruits, and they generally show increased yields. Some people worry about effects on wild flowers, insects and human health.
Is height caused by genes or the environment?
Height is caused by both genes and the environment. Genes set a potential height, but diet and lifestyle affect what is actually reached. Variation can be genetic, such as eye colour or blood group, environmental, such as a scar from an injury, or a combination of both, because the phenotype develops from the genome interacting with the environment.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026