How is antibiotic resistance an example of natural selection in action?
Explain how antibiotic resistance in bacteria evolves through natural selection and how it spreads
Antibiotic resistance evolves by natural selection: a resistance mutation lets some bacteria survive antibiotics and reproduce, so resistant populations spread, worsened by antibiotic misuse.
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What this dot point is asking
You need to explain antibiotic resistance as a worked example of natural selection, identify the selection pressure, and explain why human behaviour accelerates it. This dot point applies the mechanism from earlier in the topic.
Antibiotics and resistance
Antibiotics are drugs that kill or stop the growth of bacteria. Antibiotic resistance is the ability of some bacteria to survive a dose of antibiotic that would normally kill them. Resistance arises from random mutations in bacterial DNA that, for example, change the target the antibiotic acts on or let the bacterium break the drug down.
Crucially, the mutations arise by chance, before the antibiotic is present. The antibiotic does not create resistance; it selects for bacteria that already happen to be resistant.
Resistance as natural selection
The process follows the standard steps of natural selection exactly:
- Variation. A bacterial population varies; by chance, a few individuals carry a mutation giving resistance.
- Selection pressure. An antibiotic is applied - this is the selection pressure.
- Differential survival. Non-resistant bacteria are killed; the resistant bacteria survive.
- Reproduction. The surviving resistant bacteria reproduce (very rapidly, by binary fission), passing on the resistance allele.
- Outcome. Over successive generations the population becomes dominated by resistant bacteria. The antibiotic is no longer effective.
Because bacteria reproduce so quickly, this evolution can be observed over days, making it a clear real-world demonstration of natural selection.
How resistance spreads
Resistance can spread in two ways:
- Vertically, from parent to daughter cells as bacteria divide.
- Horizontally, between bacteria (even of different species) by transferring small rings of DNA called plasmids that carry resistance genes.
Horizontal transfer means resistance can move rapidly through bacterial populations.
Why human behaviour worsens it
Several practices increase the selection pressure and speed up resistance:
- Overuse and misuse of antibiotics (for example, for viral infections, which antibiotics do not treat).
- Not completing a course, leaving partially resistant bacteria alive to reproduce.
- Widespread use in agriculture.
Slowing resistance involves using antibiotics only when needed, completing prescribed courses, and developing new drugs.
Why bacteria evolve resistance so fast
Antibiotic resistance is the clearest real-time example of evolution partly because of how bacteria reproduce. Their generation time can be as short as twenty minutes, so a population passes through thousands of generations in the time a single human generation takes. Combined with very large population sizes, this means even rare resistance mutations appear somewhere in the population, and once they confer an advantage they can sweep through it within days. Horizontal gene transfer via plasmids accelerates this further, because a resistance gene that arose in one species can be shared with unrelated bacteria without waiting for them to reproduce. This is why resistance is observed within human lifetimes, unlike most evolutionary change.
Connecting to the wider topic
This dot point is best understood as natural selection applied to a fast-reproducing organism under a human-imposed selection pressure. The same logic explains pesticide resistance in insects and herbicide resistance in weeds. It also illustrates a key conceptual point examiners test: the environment selects from existing variation rather than directing or creating useful mutations, so the resistance allele must already be present (by chance) before the antibiotic is applied.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SACE 20186 marksPlants synthesise amino acids in a metabolic process that involves several consecutive steps. Glyphosate is a widely used weedkiller that prevents an enzyme from functioning properly in one of these steps, and this results in the death of the plant. There is growing concern that some species of plants have developed resistance to glyphosate. Describe how a species may, over time, develop resistance to a chemical that was previously lethal to that species.Show worked answer →
This 6 mark question wants the full natural selection sequence applied to chemical resistance, which is the same mechanism as antibiotic resistance in bacteria. Award roughly one mark per linked idea.
Within the species there is genetic variation, arising from random mutation, so a few individuals carry an allele that happens to give some resistance to the chemical.
These resistance mutations exist by chance before the chemical is applied; the chemical does not create them.
Applying the chemical (glyphosate) is the selection pressure. It kills the susceptible, non-resistant individuals.
The resistant individuals survive, because they are better suited to this changed environment.
The survivors reproduce and pass the resistance allele on to their offspring.
Over many generations the proportion of resistant individuals increases until most of the population is resistant, so the chemical is no longer effective. This is evolution by natural selection.
SACE 20213 marksA hospital records the percentage of Staphylococcus aureus samples resistant to a particular antibiotic rising from in 2005 to in 2020. (a) Explain, in terms of natural selection, why the percentage of resistant bacteria increased. (b) Suggest one hospital practice that could slow this increase and justify it.Show worked answer →
Three marks: two for the natural selection explanation, one for a justified practice.
(a) Increase in resistance (2 marks). The bacterial population varied, with a small proportion carrying a resistance allele from random mutation. Repeated use of the antibiotic in the hospital acted as a selection pressure, killing susceptible bacteria while resistant bacteria survived and reproduced (rapidly, by binary fission), passing on the resistance allele. Over many generations the proportion of resistant bacteria rose from to because the resistant individuals had the survival and reproductive advantage whenever the antibiotic was present.
(b) Practice to slow it (1 mark). Reducing unnecessary antibiotic use (for example, only prescribing antibiotics for confirmed bacterial infections) lowers the selection pressure, so resistant bacteria no longer have an advantage and their proportion stops rising. Improved hygiene to reduce transmission, or rotating antibiotics, are also accepted with justification.
Markers reward variation-selection-reproduction reasoning tied to the data and a valid practice linked to reduced selection pressure or transmission.