How does natural selection drive evolution, and what is the evidence?
The theory of evolution by natural selection, how variation, competition and survival of the best-adapted lead to a change in a species over time, antibiotic resistance as an example, and the evidence from fossils.
A focused CCEA GCSE Biology answer on natural selection and evolution, covering the theory of evolution by natural selection, the steps from variation to a changed species, antibiotic resistance, and the fossil evidence.
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What this dot point is asking
CCEA wants you to describe the theory of evolution by natural selection, explain the steps from variation to a changed species, use antibiotic resistance as an example, and describe the fossil evidence for evolution.
The theory of natural selection
Antibiotic resistance
Antibiotic resistance is natural selection happening now. By chance, mutation gives a few bacteria an allele for resistance. When an antibiotic is used, it kills the non-resistant bacteria, but the resistant ones survive, reproduce and pass on the resistance allele, so the population becomes resistant.
Evidence from fossils
Examples in context
- Example 1. Why finishing an antibiotic course matters
- If you stop taking an antibiotic early, the most resistant bacteria, the ones that survive longest, may still be alive. These survivors reproduce and pass on their resistance, making future infections harder to treat. Finishing the full course kills even the more resistant bacteria before they can multiply. This everyday advice is rooted in natural selection, and CCEA often asks you to explain it.
- Example 2. How new evidence supported Darwin
- When Charles Darwin proposed natural selection, the way characteristics were inherited was not understood. Later discoveries about genes, DNA and mutation explained the source of the variation that selection acts on, and the growing fossil record showed the gradual changes Darwin predicted. Today, antibiotic resistance lets us watch natural selection happen within years. Together these strands of evidence, genetics, fossils and observed selection, strongly support the theory of evolution.
- Example 3. Why extinction can happen
- Natural selection only changes a species fast enough if there is enough variation and enough time. If the environment changes very quickly, for example through a sudden loss of habitat or a new disease, a species may have no individuals with the alleles needed to survive the new conditions. With no suitable variation to be selected, the whole population can die out, which is extinction. This is the flip side of natural selection: the same process that gradually adapts a species to a changing environment cannot save it if the change is too fast or too large. CCEA links this to conservation and to the importance of biodiversity, because a varied population is more likely to contain individuals that can survive a change.
Try this
Q1. What is meant by survival of the fittest? [1 mark]
- Cue. The best-adapted individuals are most likely to survive and reproduce.
Q2. Where are fossils found? [1 mark]
- Cue. In layers of rock.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20216 marksExplain how a population of bacteria can become resistant to an antibiotic by natural selection.Show worked answer →
Six marks for the full natural-selection sequence applied to bacteria.
In a population of bacteria there is variation, caused by mutation; 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.
The resistant bacteria survive (survival of the fittest) because they are best suited to the new conditions.
The survivors reproduce and pass on the resistance allele to their offspring.
Over time the proportion of resistant bacteria increases, so the whole population becomes resistant and the antibiotic no longer works.
Markers reward variation by mutation, antibiotic kills non-resistant, resistant survive, reproduce and pass on the allele, population becomes resistant.
CCEA 20193 marksDescribe how fossils provide evidence for evolution.Show worked answer →
Three marks for what fossils are and how they show change.
Fossils are the preserved remains or traces of organisms that lived long ago, found in rock layers.
Older rocks (deeper layers) contain simpler organisms, and younger rocks contain more complex ones, showing that life has changed over time.
By comparing fossils of different ages, we can see how a group of organisms gradually changed, providing evidence for evolution.
Markers reward fossils as preserved remains, found in layers of different ages, showing gradual change over time.
Related dot points
- The difference between continuous and discontinuous variation, the genetic and environmental causes of variation, how variation data is presented, and the role of mutation in producing new variation.
A focused CCEA GCSE Biology answer on variation, covering continuous and discontinuous variation, the genetic and environmental causes of variation, how variation data is presented, and the role of mutation.
- How selective breeding chooses parents with desired characteristics over many generations, examples in crops and farm animals, and the benefits and risks including reduced variation and inbreeding.
A focused CCEA GCSE Biology answer on selective breeding, covering how parents with desired features are chosen over many generations, examples in crops and animals, and the benefits and risks including reduced variation.
- Mitosis as cell division producing two genetically identical cells for growth and repair, meiosis as division producing four genetically different gametes with half the chromosome number, and why meiosis creates variation.
A focused CCEA GCSE Biology answer on cell division, covering mitosis producing identical cells for growth and repair, meiosis producing genetically different gametes with half the chromosome number, and why meiosis creates variation.
- Pathogens as disease-causing microorganisms and how they spread, the body's first-line defences such as the skin, and the role of white blood cells in defending the body by phagocytosis and antibody production.
A focused CCEA GCSE Biology answer on defence against disease, covering pathogens and how they spread, the body's first-line defences such as the skin, and the role of white blood cells in phagocytosis and antibody production.
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A focused CCEA GCSE Biology answer on medicines and drugs, covering how antibiotics treat bacteria but not viruses, antibiotic resistance and how to reduce it, medical versus recreational drugs, and the effects of alcohol and tobacco.
Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)