Where does variation come from, and how does natural selection lead to evolution?
Genetic and environmental variation, mutation as a source of new alleles, natural selection and evolution, the evidence from fossils, selective breeding, and the evolution of antibiotic resistance.
A focused answer to the WJEC GCSE Biology section 2.4 topic on variation and evolution, covering genetic and environmental variation, mutation, natural selection and evolution, the evidence from fossils, selective breeding, and the evolution of antibiotic resistance.
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What this dot point is asking
WJEC wants you to describe the causes of variation, explain mutation as a source of new alleles, describe natural selection and evolution and the evidence from fossils, explain selective breeding, and explain the evolution of antibiotic resistance.
Causes of variation
Differences between organisms are called variation, and there are two main causes.
- Genetic variation is caused by differences in genes (alleles). It is inherited from the parents and arises through meiosis, the mixing of alleles at fertilisation, and mutation. Examples include eye colour and blood group.
- Environmental variation is caused by the surroundings, not the genes. Examples include a scar, a suntan, or a plant growing taller in better soil.
Many features are caused by a combination of both, such as height, which depends on genes and on diet.
Mutation
Most mutations have no effect or are harmful, but occasionally a mutation gives an organism an advantage. The rate of mutation is increased by ionising radiation and some chemicals.
Natural selection and evolution
Natural selection works in a clear sequence:
- Organisms in a population show variation, much of it from mutation.
- There is competition for resources such as food, and not all survive.
- Organisms with advantageous features are more likely to survive and reproduce.
- They pass on the alleles for those features to their offspring.
- Over many generations, the advantageous alleles become more common, so the population gradually changes (evolves).
Evidence from fossils
Fossils are the preserved remains or traces of organisms from long ago, found in rock. They provide evidence for evolution by showing how organisms have changed over time and how earlier forms relate to those alive today. Other evidence comes from similarities in DNA between species.
Selective breeding
Selective breeding (artificial selection) is when humans choose which organisms breed, to produce offspring with desired features. For example, a farmer breeds the cows that give the most milk, then breeds the best of their offspring, and so on over many generations. This works like natural selection, but humans, not the environment, do the selecting. A disadvantage is that it reduces genetic variation, which can increase the risk of inherited disease.
The evolution of antibiotic resistance
Antibiotic resistance is a clear modern example of natural selection. In a population of bacteria, random mutations mean a few are resistant to an antibiotic. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant ones survive and reproduce, passing on the resistance allele. Over time, the whole population can become resistant. Overusing antibiotics speeds this up, which is why doctors are careful to use them only when needed.
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 style6 marksExplain how a population of bacteria can become resistant to an antibiotic. Use the idea of natural selection.Show worked answer →
A 6-mark extended question. Mark it for a clear natural selection sequence.
There is variation in the bacteria, caused by random mutations; by chance, a few bacteria have an allele that makes them resistant to the antibiotic. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant ones survive. The surviving resistant bacteria reproduce, passing on the allele for resistance to their offspring. Over time, the proportion of resistant bacteria increases, until most of the population is resistant.
A top answer includes: variation from mutation; the antibiotic kills the non-resistant bacteria; the resistant survivors reproduce and pass on the allele; the population becomes resistant over generations. Saying the bacteria "decide" to become resistant is wrong; resistance arises by chance mutation.
WJEC style4 marksDescribe how a farmer could use selective breeding to produce cows that give more milk, and give one disadvantage.Show worked answer →
A 4-mark question.
The farmer chooses the cows that produce the most milk and breeds them together. From the offspring, they again select those that give the most milk and breed them. Repeating this over many generations gradually increases the milk yield, because the alleles for high yield become more common.
One disadvantage is that selective breeding reduces genetic variation (inbreeding), which can increase the risk of inherited diseases or make the population less able to cope with change.
Markers reward: choose the highest-yielding cows; breed them; repeat over generations; one valid disadvantage such as reduced variation. Saying it happens "in one generation" is wrong.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)