Selection and changing allele frequenciesAQA A-Level Biology: Subtopic test

1. 1
   On a hospital ward, a population of bacteria is exposed to an antibiotic. Before treatment, a very small proportion of the bacteria carried an allele that makes them resistant to the antibiotic.

   (a)

   What is the primary source of new alleles in a population?

   [1 mark]

   • AMeiosis
   • BRandom fertilisation of gametes
   • CMutation
   • DDifferential reproductive success

   (b)

   Which type of selection occurs when the antibiotic removes the non-resistant bacteria and favours the resistant extreme?

   [1 mark]

   • ADirectional selection
   • BStabilising selection
   • CDisruptive selection
   • DRandom selection

   (c)

   Explain how the frequency of the resistance allele in the bacterial population increases after the antibiotic is used.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In human populations, babies with a birth weight close to the population mean of about 3.3 kg have the highest rate of survival. Babies with a very low or very high birth weight have a higher death rate.

   (a)

   Which type of selection is acting on birth weight?

   [1 mark]

   • ADirectional selection
   • BStabilising selection
   • CDisruptive selection
   • DArtificial selection

   (b)

   If this selection pressure continues for many generations, what is the most likely effect on birth weight in the population?

   [1 mark]

   • AThe mean birth weight increases steadily
   • BThe mean birth weight decreases steadily
   • CThe distribution develops two separate peaks
   • DThe range of birth weights narrows around the mean

   (c)

   Explain the effect of this type of selection on the frequencies of the alleles affecting birth weight.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   On a small island, seed-eating birds feed on seeds of different sizes. Beak depth varies widely between individual birds and is partly inherited. During a severe drought, the plants producing small, soft seeds died, leaving mainly large, hard seeds. After the drought, the mean beak depth of the next generation of birds was greater than before.

   (a)

   Explain how the drought led to an increase in the mean beak depth of the birds in the next generation.

   [3 marks]

   (b)

   Explain why the individual birds in the population show a wide range of beak depths.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Beetles on a hillside show body colours ranging from very pale to very dark. The hillside is a patchwork of pale sandy soil and dark volcanic rock. Birds eat the beetles that they can see easily. Beetles of intermediate colour are easy to see on both backgrounds.

   (a)

   Explain how the selection acting on the beetles would change the allele frequencies and the distribution of body colour over many generations.

   [6 marks]

   (b)

   Using this example, explain how evolutionary change over a very long period has resulted in a great diversity of species.

   [6 marks]

   Total for question 4: 12 marks

End of questions