IGCSE Biology / Natural selection and evolution
Antibiotic resistance and the peppered moth revision notes
Edexcel International GCSE Biology (4BI1), chapter 19: Natural selection and evolution, specification point 3.39. Free lesson.
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Natural selection explains how antibiotic-resistant bacteria and dark peppered moths become more common. In both cases, variation exists before the environment changes, and individuals with a helpful characteristic survive and reproduce more.
How antibiotic resistance develops
Resistance begins with a random mutation in one bacterium, before an antibiotic is used. The antibiotic then kills bacteria that are not resistant. The resistant bacterium survives and reproduces quickly, so its resistance allele becomes common and the antibiotic eventually stops working. The mutation is not caused by the antibiotic. It is already present when the antibiotic is first used. Bacteria reproduce rapidly, so resistance can spread through a population in a short time.
How the peppered moth changed
Before the industrial revolution, most peppered moths were pale and camouflaged against pale lichen on tree trunks. A dark form already existed, but it was rare. Soot killed the lichen and darkened tree trunks, changing which moths were easiest for birds to see. Pale moths stood out against dark bark and were eaten more often. Dark moths were better camouflaged, so they survived and reproduced more, passing on the allele for dark colouring. Within decades, dark moths became common in industrial areas. When the air became cleaner, pale moths became common again.
What makes resistance spread faster
Antibiotic use creates selection pressure: susceptible bacteria are killed, while more resistant bacteria survive to reproduce. Some human actions increase this pressure or help resistance spread. If a course is stopped early, the most susceptible bacteria have died first, but more resistant bacteria may remain and reproduce. Antibiotics do not treat viruses. However, they can still select for resistance in bacteria in the body. Routine antibiotic use in farm animals creates constant selection pressure. Resistant bacteria can then pass to humans. Bacteria reproduce quickly and can pass…
How resistance can be slowed
Reducing unnecessary exposure to antibiotics means there is less selection pressure favouring resistant bacteria. These actions help slow the spread of resistance. Finish every course of antibiotics. Prescribe antibiotics only when they are needed, not for viral infections. Avoid routine use of antibiotics in farming. Develop new antibiotics.