(i) how evolution in some species has implications for human populations.

The idea

Evolution by natural selection changes allele frequencies in populations over generations. In some species this directly affects people. When bacteria are exposed to antibiotics, resistant variants survive and reproduce, so resistance spreads and infections become harder to treat. Pests such as insects evolve resistance to pesticides, reducing crop yields. Pathogens and vectors can shift their ranges or host preferences as selection pressures change, altering disease risk. Understanding these evolutionary processes lets humans manage resistance, for example by completing prescribed antibiotic courses and rotating pesticides. The key idea is that the selection pressure applied by humans acts as an agent of natural selection, and the resulting heritable change has practical consequences for health, agriculture and disease control.

What earns the marks

• Evolution by natural selection changes allele frequencies in a population over generations.
• Antibiotic use acts as a selection pressure: resistant bacteria survive, reproduce and pass on resistance alleles, so resistant strains become more common.
• Pesticide use selects resistant insects, reducing the effectiveness of chemical control in agriculture.
• Changes in pathogen or vector populations can alter the spread and risk of human disease.
• Human practices (such as antibiotic prescribing or pesticide rotation) can be managed to slow the evolution of resistance.

Common mistakes and how to put them right

• Thinking individual bacteria 'decide' to become resistant.

  Put it right: Resistance arises from pre-existing variation, and selection increases the frequency of resistant alleles in the population.

• Assuming evolution only happens over millions of years.

  Put it right: In fast-reproducing organisms such as bacteria, measurable evolutionary change can occur in a short time.

• Confusing the origin of variation with selection.

  Put it right: Mutation generates variation, while the antibiotic or pesticide selects which variants survive.

Exam tips

• Link each example explicitly to the mechanism: variation, selection pressure, differential survival and reproduction, heritable change.
• Use a named, concrete example such as antibiotic resistance in bacteria rather than a vague statement that 'evolution affects humans'.
• Check the question wording: if it asks how evolution has implications for humans, state the consequence (for example treatment failure or crop loss) as well as the mechanism.