Darwin, natural selection and antibiotic resistance — Edexcel GCSE Combined Science
How evolution by natural selection works, and how antibiotic-resistant bacteria are evidence for Darwin's theory.
Revision notes
2 short notes, in the order of the specification. Each one in short:
Evolution is a change in the inherited characteristics of a population over many generations, and natural selection explains how it happens. Individuals show genetic variation, caused by mutation, and compete for resources. Those best suited to their environment are more likely to survive and reproduce, passing on their alleles, so advantageous alleles become more common over generations.
Antibiotic resistance is an example of natural selection seen within a human lifetime. A mutation gives one bacterium an allele for resistance. The antibiotic kills the non-resistant bacteria, but the resistant one survives, reproduces and passes on the allele. Over time most of the population is resistant, so the antibiotic no longer works against the infection.
Exam questions
4 exam-style questions (14 marks), each with its mark scheme.
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7 cards: flip them, mark what you knew, and practise the rest.
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The whole of natural selection and genetic modification on one page, so you can see where this subtopic fits.
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Quick check questions
What causes new alleles to appear in a population?
Show answer
Mutation.
Give the four main steps of natural selection in order.
Show answer
Variation, competition so some survive and reproduce, alleles passed on, proportion with the useful allele increases.
In antibiotic resistance, what is the selection pressure?
Show answer
The antibiotic.
Why do populations of bacteria change quickly?
Show answer
Bacteria reproduce very rapidly, so the resistance allele spreads in a short time.
Frequently asked questions
What are the 4 stages of natural selection?
The four stages of natural selection are variation, competition, survival and inheritance. Individuals show variation in their alleles, caused by mutation. More offspring are born than can survive, so they compete. Those best suited to their environment survive and reproduce, then pass on their advantageous alleles, which become more common over generations.
How does natural selection lead to evolution?
Natural selection leads to evolution because individuals with advantageous characteristics are more likely to survive, reproduce and pass on their alleles. In each generation, the proportion of the population with those alleles increases. Over many generations this changes the inherited characteristics of the population, and that change is evolution.
How do bacteria become resistant to antibiotics?
Bacteria become resistant to antibiotics through mutation and natural selection. A random mutation gives one bacterium an allele for resistance. The antibiotic kills the non-resistant bacteria, but the resistant one survives, reproduces with less competition and passes on the allele. Antibiotics do not cause the mutation; they select the resistant bacteria.
What does survival of the fittest mean?
Survival of the fittest means that the individuals best suited to their environment are the most likely to survive and reproduce. In natural selection, 'fittest' does not mean strongest. It means having characteristics that give an advantage in that environment, so these individuals pass on their alleles to the next generation.
Why is antibiotic resistance evidence for evolution?
Antibiotic resistance is evidence for evolution because every step of natural selection can be observed within a human lifetime. There is variation from mutation, the antibiotic is a selection pressure, the resistant bacteria survive and reproduce, and they pass on the resistance allele. Bacteria reproduce very quickly, so the change happens fast.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026