Natural selection and adaptationAQA A-Level Biology: Mind map

Exam questions on Natural selection and adaptation

1. A hospital ward has a patient infected with a bacterium. When the patient is first treated with an antibiotic, almost all the bacteria are killed. A few bacteria carry an allele for an enzyme that breaks down the antibiotic. Over several weeks of repeated treatment, most of the bacteria in the patient are found to carry this allele.
   Explain why the proportion of bacteria carrying the allele increased during the treatment.2 marks
2. A study of a large number of human births recorded the birth weight of each baby and whether the baby survived the first month. Mortality was lowest for babies with a birth weight of about 3.5 kg. It was 1.2% at 3.5 kg, but 18% for babies of 1.5 kg, and also much higher for babies far heavier than 3.5 kg.
   Explain how selection keeps the mean birth weight of humans about the same over many generations.2 marks
3. A student investigates the effect of three antibiotics on a bacterium. She spreads a culture of the bacterium over the surface of nutrient agar in a Petri dish. She then places four sterile paper discs on the agar: one soaked in antibiotic A, one in B, one in C, and a control disc soaked in sterile water. After incubation, the diameters of the clear zones around the discs are: A 12 mm, B 20 mm, C 0 mm and the control 0 mm.
   Describe three precautions the student should take to reduce the risk of contamination and of culturing pathogens.3 marks