What causes variation, and how does natural selection lead to evolution?
Genetic and environmental variation, the theory of evolution by natural selection, and the evidence for evolution.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on variation and evolution, covering genetic and environmental variation, evolution by natural selection, and the evidence for evolution including fossils and antibiotic resistance.
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What this dot point is asking
WJEC Double Award Unit 4 wants you to describe genetic and environmental variation, explain evolution by natural selection, and describe the evidence for evolution.
Variation
- Genetic variation: caused by the alleles inherited from parents and by mutations (random changes to DNA). Examples include natural eye colour and blood group.
- Environmental variation: caused by an organism's surroundings, such as a scar, a suntan, or a plant growing taller with more light.
- Both: many features, such as height and body mass, are affected by both genes and environment.
Sexual reproduction and mutation are the main sources of genetic variation, which is important for natural selection.
Evolution by natural selection
A mutation can produce a new, useful characteristic that natural selection then spreads through the population.
Evidence for evolution
- Fossils: the preserved remains of organisms in rock show how living things have changed over time, from simple to more complex forms.
- Antibiotic resistance: bacteria can become resistant to antibiotics by natural selection, which scientists can observe happening, providing direct evidence.
These pieces of evidence support the theory that all species have evolved from earlier ones.
Antibiotic resistance as natural selection
Antibiotic resistance is a clear, modern example. By chance (from a mutation), some bacteria are resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive and reproduce, passing on the resistance. Over time the whole population becomes resistant. This is why doctors avoid overusing antibiotics, and it shows natural selection happening quickly because bacteria reproduce so fast.
Selective breeding
Humans can speed up changes in a species by selective breeding (artificial selection), which is similar to natural selection but with people doing the choosing. Breeders choose the individuals with the desired characteristic (such as cows that give more milk or wheat with a bigger yield) and breed them together; they then repeat this over many generations, so the useful characteristic becomes stronger. Selective breeding has given us most farm animals and crop plants, but it can reduce variation and cause health problems if overdone. Comparing selective breeding with natural selection (who does the choosing) is a common exam point.
Try this
Q1. Give one example of environmental variation. [1 mark]
- Cue. Any one of: a scar, a suntan, a plant grown taller by more light.
Q2. What is the random change to DNA that creates new variation called? [1 mark]
- Cue. A mutation.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style5 marksExplain how a population of bacteria can become resistant to an antibiotic, using the theory of natural selection.Show worked answer →
A Unit 4 extended question worth 5 marks. Reward: there is variation in the bacteria, caused by mutations, so some are resistant to the antibiotic by chance (1); when the antibiotic is used, the non-resistant bacteria are killed (1); the resistant bacteria survive and reproduce (1), passing on the resistance allele to their offspring (1); over time the whole population becomes resistant (1). Markers credit variation from mutation, the survival of the resistant ones, reproduction and passing on the gene, and the population changing. A common error is to say bacteria "choose" to become resistant.
WJEC style3 marksGive two causes of variation and state whether each is genetic, environmental or both.Show worked answer →
A Unit 4 recall question. Reward: genetic variation is caused by different alleles inherited from parents (and mutations), for example natural eye colour (1); environmental variation is caused by surroundings, for example a scar or being taller through diet (1); some characteristics, such as height or weight, are caused by both genes and environment (1). Markers credit a genetic and an environmental cause correctly identified. A common error is to call an inherited feature environmental.
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