Reproduction and Inheritance · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
Natural selection explanations and antibiotic resistance as an application are tested in almost every series.
is one of the most important real-world examples of natural selection, and arguably the most serious medical problem facing modern healthcare.
Antibiotics are drugs that kill bacteria (or stop them growing). They work by targeting features of bacterial cells that human cells do not have:
Antibiotics have saved millions of lives since the discovery of penicillin in 1928. Before antibiotics, even a minor infection could be fatal.
In any population of bacteria, there is genetic from random mutations. Occasionally a mutation produces a protein that makes the bacterium immune to a particular antibiotic (for example, by changing the shape of the antibiotic's target so the antibiotic can no longer bind).
When the bacterial population is exposed to that antibiotic:
Because bacteria reproduce so quickly, this whole process can happen in days or weeks, not the centuries it might take in a slower-reproducing species.
Several human behaviours make resistance worse:
Explaining how bacteria evolved resistance to an antibiotic
What comes up: questions ask you to explain how bacteria have evolved so that an antibiotic is less effective — typically 4 marks.
Write (four marks): (1) A random mutation in some bacteria produced an allele conferring resistance to the antibiotic. (2) When the antibiotic was used, non-resistant bacteria were killed but the resistant bacteria survived. (3) The resistant bacteria reproduced rapidly, passing on the resistance allele to their offspring. (4) Over subsequent generations, the frequency of the resistance allele increased so the majority of the bacterial population became resistant.
Watch out: do not write that bacteria "became immune" to the antibiotic — this phrasing is not credited. The resistance mutation arose by chance before the antibiotic was introduced; the antibiotic acted as the selection pressure, it did not cause the mutation.