Genetic driftAQA A-Level Biology: Subtopic test

1. 1
   A conservation biologist monitors a small island population of 12 flightless birds. A gene for feather pattern has two alleles, F and f, which have no effect on survival or reproduction. Over three generations the frequency of allele f changed from 0.50 to 0.29 and then to 0.00.

   (a)

   Which statement defines genetic drift?

   [1 mark]

   • AA change in allele frequency caused by the survival of the best adapted individuals
   • BA change in allele frequency caused by new alleles arising by mutation
   • CThe movement of alleles between populations by migration
   • DA random change in allele frequency caused by chance in which alleles are passed on

   (b)

   The biologist also studies a population of 1200 birds on a nearby island, with the same alleles at the start. Which statement is most likely to be correct?

   [1 mark]

   • AThe allele frequency changes faster in the 1200 population because larger populations have more mutations
   • BThe allele frequency changes less in the 1200 population because chance events have a smaller effect on a large gene pool
   • CThe allele frequency changes by the same amount in both populations because drift acts equally on all populations
   • DThe allele frequency does not change in the 1200 population because large populations are never affected by selection

   (c)

   Explain why the frequency of allele f fell to zero even though neither allele gave an advantage.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two isolated populations of voles carry the same gene with alleles Z and z, which have no effect on survival or reproduction. At the start, the frequency of allele Z is 0.50 in both populations. Population X has 50 voles and population Y has 5000 voles. After 20 generations the frequency of Z is 1.00 in population X and 0.49 in population Y.

   (a)

   Which statement best explains the difference between the two populations after 20 generations?

   [1 mark]

   • AChance has a greater effect on allele frequency in a small population than in a large one
   • BNatural selection acted more strongly on allele z in population X
   • CPopulation Y had a higher mutation rate
   • DAllele Z gave an advantage in population X but not in population Y

   (b)

   Which statement correctly compares genetic drift with natural selection?

   [1 mark]

   • AGenetic drift always increases the frequency of advantageous alleles
   • BGenetic drift and natural selection always change allele frequencies in the same direction
   • CGenetic drift can change the frequency of an allele whether or not it is advantageous, whereas selection depends on survival advantage
   • DNatural selection changes allele frequencies at random, whereas genetic drift is not random

   (c)

   In each population, one vole with the genotype zz dies by chance. Calculate the percentage of all the copies of the gene lost from each population, and explain what this shows about genetic drift.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A population of 2000 field mice lives in a meadow. A flood kills all but 10 of the mice. The deaths were not related to the phenotypes of the mice. The survivors then breed and the population recovers to 2000 mice.

   (a)

   Explain how the flood could have changed the allele frequencies in the population that recovered.

   [3 marks]

   (b)

   Explain why the recovered population might be less able than the original population to respond to a new disease.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A remote community of 200 people was founded 300 years ago by 12 settlers. A recessive allele that causes a rare metabolic disorder has a frequency of 0.12 in the community. In the national population of 60 million people the frequency of the same allele is 0.002. There is no evidence that carriers of the allele have any advantage.

   (a)

   Explain why genetic drift is important only in small populations, and use this to explain the difference between the allele frequencies in the community and in the national population.

   [6 marks]

   (b)

   Evaluate the relative importance of genetic drift and natural selection in determining the allele frequency in the community and in the national population.

   [6 marks]

   Total for question 4: 12 marks

End of questions