Book a tutor
SearchResourcesRevision notesFlashcardsMind mapsExam questionsTopic testsPracticalsGCSE Biology coursesAQA GCSE Biology (8461)AQA GCSE Combined Science: Trilogy (8464)Edexcel GCSE Biology (1BI0)Edexcel GCSE Combined Science (1SC0)IGCSE Biology coursesCambridge IGCSE Biology (0610)Cambridge IGCSE Combined Science (0653)Edexcel International GCSE Biology (4BI1)Edexcel International GCSE Science (Double Award) (4SD0)MoreAboutContactBook a tutor

Antibiotic-resistant bacteria

Variation and selection · Selection · note 2 of 3

Spec 18.3.5
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
Download all Selection notes (PDF)Download all notes (PDF)4 pages

Antibiotic-resistant bacteriaSpec 18.3.5

In short

Antibiotic-resistant bacteria are an example of natural selection. A random mutation produces an allele that makes a bacterium resistant. When the antibiotic is used, it kills the non-resistant bacteria, but the resistant bacterium survives, reproduces and passes on the resistance allele. Bacteria reproduce very quickly, so a resistant strain can become common in a short time.

The development of strains of antibiotic-resistant bacteria is an example of natural selection.

  1. Among a population of bacteria, a random mutation produces a new allele that makes one bacterium resistant to an antibiotic.
  2. The antibiotic is used. It kills the non-resistant bacteria.
  3. The resistant bacterium survives because it is better adapted in the presence of the antibiotic, and it has less competition.
  4. It reproduces, and passes the resistance allele to its offspring.
  5. Over time, most of the population is resistant and the antibiotic no longer works.

Bacteria can reproduce very quickly, so a resistant strain can become common in a short time. Overuse of antibiotics increases the chance that resistant strains are selected.

Common mistake:

Say the bacteria are resistant, not the person. The antibiotic does not cause the mutation; it selects the bacteria that already have the resistance allele.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How does natural selection lead to evolution?

Supplement Natural selection leads to adaptation of a population over many generations. There is genetic variation, and more offspring are produced than the environment can support, so there is a struggle for survival. Better adapted individuals survive and reproduce, passing on their alleles. The helpful alleles become more common, so the population becomes more suited to its environment.

What are the stages of natural selection?

Natural selection has five stages. There is genetic variation within populations. Organisms produce more offspring than the environment can support. There is a struggle for survival, including competition for food, water and mates. Better adapted individuals have a greater chance of surviving and reproducing. They pass on their alleles, so helpful alleles become more common.

How does antibiotic resistance show natural selection?

Supplement Antibiotic resistance shows natural selection because the antibiotic selects bacteria that already have a resistance allele. A random mutation makes one bacterium resistant. The antibiotic kills the non-resistant bacteria, and the resistant one survives, reproduces and passes on the allele. Over time most of the population is resistant and the antibiotic no longer works.

All 5 questions on Selection