Evolution & Natural Selection

IGCSE Edexcel Biology
3.34–3.39 Natural selection, evolution, and antibiotic resistance
Key Concepts: Natural selection acts on variation in a population. Individuals with advantageous characteristics are more likely to survive and pass on their alleles. Over many generations, this leads to evolution. Antibiotic resistance is a modern example of natural selection.

Section A — Variation and Mutation

1. State two sources of genetic variation in a population. [2]
2. Define mutation. State two factors that can increase the rate of mutation. [3]

Section B — Natural Selection

3. Explain the process of natural selection in four steps. [4]
4. The peppered moth exists in light and dark forms. Before industrialisation, the light form was more common. After industrialisation, the dark form became more common in urban areas. Explain this change using natural selection. [5]
5. Define evolution. [2]

Section C — Antibiotic Resistance

6. Define antibiotic. [2]
7. Explain how antibiotic resistance develops in a population of bacteria. [5]
8. State two actions that humans should take to reduce the spread of antibiotic-resistant bacteria. [2]
9. Explain why a patient with a bacterial infection should always complete the full course of antibiotics. [3]

Total marks: 28

Mark Scheme

1. Any two: mutation (random change in DNA); meiosis/sexual reproduction (shuffling of alleles); crossing over; random fertilisation [2]
2. A random change in the sequence of bases in DNA / a change in the genetic material [1]; any two: ionising radiation (UV, X-rays); certain chemicals (mutagens); some viruses [2] [3]
3. There is variation in a population (due to random mutation) [1]; the environment creates a selection pressure [1]; individuals with advantageous characteristics survive and reproduce more (survival of the fittest) [1]; advantageous alleles are passed to offspring and become more frequent in the population [1] [4]
4. Before industrialisation, light bark on trees = light moth better camouflaged / dark moth more visible to predators [1]; dark moths were eaten more / light moths survived and reproduced more [1]; after industrialisation, soot darkened tree bark [1]; dark moths were now better camouflaged and survived / light moths were now more visible and eaten [1]; dark allele became more common in the population [1] [5]
5. A change in the inherited characteristics (allele frequencies) of a population over many generations [2]
6. A chemical that kills or inhibits the growth of bacteria [2]
7. A mutation occurs in a bacterium giving it resistance to an antibiotic [1]; when antibiotics are used, non-resistant bacteria are killed [1]; the resistant bacterium survives and reproduces [1]; resistance alleles are passed to offspring [1]; over time the resistant strain becomes dominant in the population [1] [5]
8. Any two: only use antibiotics when necessary (not for viral infections); always complete the full course; avoid giving antibiotics to animals unless needed; develop new antibiotics [2]
9. If the course is not completed, some bacteria (possibly slightly more resistant) may survive [1]; these reproduce and the more resistant bacteria become more common [1]; eventually a fully resistant strain develops that the antibiotic cannot treat [1] [3]