Natural selection and adaptationAQA A-Level Biology: Subtopic test

1. 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.

   (a)

   How did the allele for the antibiotic-breaking enzyme most probably arise in the bacterial population?

   [1 mark]

   • AThe antibiotic caused the bacteria to mutate so they could survive it
   • BThe bacteria chose to develop the allele when they were exposed
   • CA random mutation occurred in the DNA of a bacterium
   • DThe bacteria that were killed passed the allele to the survivors

   (b)

   Which type of selection is shown by the bacteria exposed to the antibiotic?

   [1 mark]

   • ADirectional selection
   • BStabilising selection
   • CArtificial selection
   • DRandom genetic drift

   (c)

   Explain why the proportion of bacteria carrying the allele increased during the treatment.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which type of selection acts on birth weight in humans, according to the study?

   [1 mark]

   • ADirectional selection towards heavier babies
   • BDirectional selection towards lighter babies
   • CSelection with no effect on the range of weights
   • DStabilising selection

   (b)

   How many times greater was the mortality of babies of 1.5 kg than that of babies of 3.5 kg?

   [1 mark]

   • A6.7
   • B16.8
   • C15
   • D0.07

   (c)

   Explain how selection keeps the mean birth weight of humans about the same over many generations.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe three precautions the student should take to reduce the risk of contamination and of culturing pathogens.

   [3 marks]

   (b)

   Calculate the area of the clear zone around the discs for A and for B, using area = $\pi r^2$. Use the results, and the control, to evaluate which antibiotic is most effective against this bacterium.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   On a small island, a population of ground-feeding birds eats seeds. Beak depth varies between individuals and is controlled by several genes. After a severe drought, only large, hard-cased seeds were left on the island. Over the next six years, the mean beak depth of the population rose from 9.2 mm to 10.1 mm.

   (a)

   Explain how selection could have produced the change in mean beak depth in the population of birds.

   [6 marks]

   (b)

   Calculate the percentage increase in mean beak depth. Explain how selection on beak depth leads to adaptation of the population, and how adaptation and selection contribute to the diversity of living organisms. Give one physiological and one behavioural adaptation, other than beak depth, that could help the birds to survive a drought.

   [6 marks]

   Total for question 4: 12 marks

End of questions