How does evolution by natural selection work, and what evidence supports it?
Describe the work of Darwin and Wallace and explain the theory of evolution by natural selection, including how the emergence of resistant organisms such as antibiotic-resistant bacteria supports the theory.
A focused answer to Edexcel GCSE Biology 4.1B to 4.3, covering the work of Darwin and Wallace, the theory of evolution by natural selection, and how antibiotic-resistant bacteria provide evidence for it.
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What this dot point is asking
Edexcel statements 4.1B to 4.3 want you to describe the work of Darwin and Wallace (4.1B is Biology only), explain the theory of evolution by natural selection, and explain how the emergence of resistant organisms, especially antibiotic-resistant bacteria, supports the theory. The mechanism (variation, survival, reproduction, change over generations) is the heart of it.
Darwin and Wallace
The idea was controversial at first, partly because the mechanism of inheritance was not yet understood (Mendel's work came later) and partly because it challenged existing beliefs. As genetics and fossil evidence accumulated, natural selection became the accepted explanation for the diversity of life.
The theory of natural selection
The mechanism has four steps:
- Variation: individuals in a species differ, and these differences arise from mutations in their DNA.
- Survival: in any environment, individuals with features that suit it (such as better camouflage or faster running) are more likely to survive competition, predators and disease.
- Reproduction: the survivors reproduce and pass on the alleles for the helpful features.
- Change over generations: over many generations, the helpful alleles become more common, so the whole species gradually changes.
Antibiotic resistance as evidence
Antibiotic-resistant bacteria are one of the clearest modern examples of natural selection, because it happens fast enough to observe:
- A bacterial population varies because of random mutations, and by chance a few bacteria are resistant to an antibiotic.
- When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive.
- The survivors reproduce rapidly, passing on the resistance allele.
- Over generations the resistant form spreads until the whole population is resistant, and the antibiotic no longer works.
This is why doctors avoid overusing antibiotics and why patients are told to finish the full course: stopping early leaves the more resistant bacteria alive to multiply.
Try this
Q1. State the source of the variation that natural selection acts on. [1 mark]
- Cue. Random mutations in DNA (which create new alleles).
Q2. Explain why finishing a full course of antibiotics helps prevent resistance. [2 marks]
- Cue. Finishing the course kills the more resistant bacteria too; stopping early leaves them alive to survive and reproduce, spreading resistance.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20196 marksExplain how a population of bacteria can become resistant to an antibiotic. Use the theory of evolution by natural selection in your answer.Show worked answer →
A 6-mark extended-response question rewards a full natural-selection sequence applied to bacteria.
- There is variation in the bacterial population, caused by random mutations.
- By chance, a few bacteria have a mutation that makes them resistant to the antibiotic.
- When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive.
- The surviving resistant bacteria reproduce (by binary fission), passing on the resistance allele.
- Over many generations the proportion of resistant bacteria increases, until the whole population is resistant.
Markers reward variation by mutation, survival of the resistant individuals, reproduction passing on the allele, and the change over generations. Saying the bacteria choose to become resistant, or develop resistance because they need it, is a Lamarckian error and loses marks.
Edexcel 20212 marksCharles Darwin and Alfred Wallace both contributed to the theory of evolution by natural selection. State why a scientific theory is more accepted when more than one scientist gathers evidence for it.Show worked answer →
A 2-mark question rewards understanding how science builds confidence.
When independent scientists, such as Darwin and Wallace, reach the same conclusion from separate evidence, the theory is supported by more than one source, so it is less likely to be due to a single mistake or bias.
This independent agreement, together with evidence that others can repeat and check, makes the scientific community more confident the theory is correct. Markers reward the idea of independent evidence increasing confidence. Saying simply that two scientists are better than one, without the reasoning, scores less.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)