Cotton plants are attacked by an insect pest. Farmers spray the cotton crops with a pesticide to kill the insects.
After several years of spraying, the pesticide killed fewer of the insects.
Explain how the insect population became resistant to the pesticide.
44 exam-style questions on Edexcel GCSE Biology 4.1 Evolution and natural selection, covering 4.1.1 Work of Darwin and Wallace and 4.1.2 Resistant organisms and evolution. Each one has a worked solution and a mark scheme showing where the marks go.