How does evolution by natural selection happen, and what evidence supports it?
Darwin's theory of evolution by natural selection, the role of variation and mutation, evidence from fossils and antibiotic resistance, and how new species form.
A focused answer to Edexcel GCSE Combined Science Topic 4 (CB4), covering Darwin's theory of evolution by natural selection, the role of variation and mutation, the evidence from fossils and antibiotic-resistant bacteria, and how new species form.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel wants you to state Darwin's theory of evolution by natural selection, explain the part played by variation and mutation, describe the evidence from fossils and antibiotic-resistant bacteria, and outline how new species form.
Darwin's theory of natural selection
The logic of natural selection is a fixed chain you should be able to reproduce:
- Variation. Individuals in a population differ, partly because of mutations that create new alleles.
- Competition and survival. Organisms compete for limited resources; those with advantageous characteristics are more likely to survive.
- Reproduction. Survivors reproduce and pass the advantageous alleles to their offspring.
- Inheritance over generations. Over many generations, the advantageous alleles become more common, so the population changes.
The role of mutation
Mutations are random changes to DNA that produce new alleles. Most have no effect, but occasionally a mutation gives an advantage in a particular environment. Natural selection then increases the frequency of that allele. Without mutation there would be no new variation for selection to act on.
Evidence for evolution
- Fossils. Fossils in dated rock layers show how organisms have changed over millions of years, with simpler forms in older rocks. Gaps exist because soft-bodied organisms rarely fossilise.
- Antibiotic-resistant bacteria. Bacteria evolve fast enough to observe. Resistant strains such as MRSA arise by mutation and spread when antibiotics kill the non-resistant bacteria, leaving the resistant ones to reproduce.
How new species form
If two populations of a species become isolated (for example by a mountain range or sea), they experience different conditions. Different mutations and selection pressures change each population in different ways. Over a very long time they may become so different that they can no longer interbreed to produce fertile offspring, at which point they are separate species.
Try this
Q1. State the source of the new alleles that natural selection acts on. [1 mark]
- Cue. Mutations.
Q2. Give one piece of evidence for evolution. [1 mark]
- Cue. The fossil record, or the development of antibiotic-resistant bacteria.
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 marked on a clear, logical sequence.
Within a population of bacteria there is variation, caused by random mutations, so a few bacteria happen to have an allele that makes them resistant to the antibiotic. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive (they are better adapted to this environment). The surviving resistant bacteria reproduce, passing on the resistance allele to their offspring. Over many generations the proportion of resistant bacteria increases until most of the population is resistant.
Markers reward the chain: variation from mutation, survival of the resistant (the advantageous allele), reproduction, and inheritance of the allele leading to a resistant population. This is the model answer for natural selection applied to a new context.
Edexcel 20213 marksDescribe two pieces of evidence that support the theory of evolution.Show worked answer →
A 3-mark describe question.
Fossils provide a record of how organisms have changed over millions of years; older rock layers contain simpler organisms and newer layers show forms more like those alive today (1 mark, with detail for a second). The development of antibiotic-resistant bacteria is evidence of evolution happening now, because we can observe a population changing as the resistant forms become more common (1 mark). Other accepted evidence includes similarities in the anatomy of related species.
Markers reward two distinct, correctly explained pieces of evidence, with fossils and antibiotic resistance being the most reliable choices.
Related dot points
- Selective breeding and its uses and risks, genetic engineering and the steps involved, the use of genetically modified organisms, and the benefits and concerns of GM technology.
A focused answer to Edexcel GCSE Combined Science Topic 4 (CB4), covering selective breeding and its uses and risks, the steps of genetic engineering, the use of genetically modified organisms, and the benefits and concerns of GM technology.
- Monohybrid inheritance and Punnett squares, dominant and recessive alleles, predicting ratios and probabilities, genetic variation and the causes of variation, and mutations.
A focused answer to Edexcel GCSE Combined Science Topic 3 (CB3), covering monohybrid inheritance with Punnett squares, dominant and recessive alleles, predicting ratios and probabilities, the causes of genetic and environmental variation, and mutations.
- DNA, genes and chromosomes, the genome, sexual and asexual reproduction, meiosis and the production of gametes, and the key genetic terms.
A focused answer to Edexcel GCSE Combined Science Topic 3 (CB3), covering DNA, genes and chromosomes, the genome, sexual and asexual reproduction, meiosis and the production of gametes, and the key genetic terms used in inheritance.
- Levels of organisation in an ecosystem, abiotic and biotic factors, interdependence and competition, the carbon, water and nitrogen cycles, and the effect of human activity on biodiversity.
A focused answer to Edexcel GCSE Combined Science Topic 9 (CB9), covering levels of organisation in an ecosystem, abiotic and biotic factors, interdependence and competition, the carbon, water and nitrogen cycles, and human effects on biodiversity.
Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)