Explain how antibiotic resistance can develop in a bacterial population through natural selection.

IB DP Biology Standard Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Penicillin was introduced as a clinical antibiotic in the 1940s. Within years of its widespread use, strains of Staphylococcus aureus resistant to penicillin were identified in hospitals. Today, methicillin-resistant Staphylococcus aureus (MRSA) poses a significant global health threat.

Model answer (4 marks)

Variation exists within the bacterial population, with some individuals carrying heritable mutations that confer resistance to the antibiotic.
The antibiotic acts as a selection pressure, causing differential survival – resistant bacteria survive and reproduce while non‑resistant bacteria die.
Resistant bacteria reproduce and pass the resistance allele(s) to offspring, increasing the frequency of the resistance trait in the population over successive generations.
Over many generations the population becomes predominantly resistant, illustrating an evolutionary change driven by natural selection.

Examiner tips

Use the word ‘variation’ and ‘heritable mutations’ to show genetic basis. Explain the antibiotic as a selection pressure and the differential survival. Show how the allele frequency rises over generations. Link the final point to evolutionary change by natural selection.

Common mistakes

Confusing mutation with random mutation only, ignoring heritability. Failing to mention that the antibiotic is the selective pressure. Not linking the increase in allele frequency to evolutionary change.

Mark scheme (4 marks)

Variation exists within the bacterial population, with some individuals carrying heritable mutations that confer resistance to the antibiotic.
The antibiotic acts as a selection pressure, causing differential survival — resistant bacteria survive and reproduce while non-resistant bacteria die.
Resistant bacteria reproduce and pass the resistance allele(s) to offspring, increasing the frequency of the resistance trait in the population over successive generations.
Over many generations, the population becomes predominantly resistant, illustrating an evolutionary change driven by natural selection.

Key terms in this question

natural selection · antibiotic resistance