Gene pools and selectionEdexcel A-Level Biology B: Subtopic test

1. 1
   In humans, babies with a very low or a very high birth mass have a higher death rate than babies with a birth mass close to the average, which is about 3.3 kg. The alleles that influence birth mass have remained at similar frequencies in the population over many generations.

   (a)

   Which type of selection is acting on birth mass?

   [1 mark]

   • ADirectional selection
   • BDisruptive selection
   • CGenetic drift
   • DStabilising selection

   (b)

   What is the effect of this selection on the distribution of birth mass in the population?

   [1 mark]

   • AThe mean increases and the range stays the same
   • BThe mean stays about the same and the range of birth masses narrows
   • CTwo peaks form, one at each extreme
   • DThe mean decreases and the range widens

   (c)

   Explain how selection keeps the allele frequencies for birth mass similar from one generation to the next.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   On an island, a population of finches feeds on seeds. Small soft seeds and large hard seeds are both plentiful, but seeds of medium size are scarce. Birds with small beaks are efficient at eating small seeds and birds with large beaks are efficient at cracking large seeds. Birds with medium-sized beaks cannot feed efficiently on either.

   (a)

   Which change in the distribution of beak depth is expected over many generations?

   [1 mark]

   • ATwo peaks, at small and at large beak depths, with fewer medium beaks
   • BOne peak that moves towards a larger beak depth
   • CA narrower single peak at medium beak depth
   • DNo change, because beak depth is not inherited

   (b)

   After many generations, small-beaked and large-beaked finches can no longer interbreed successfully. What has occurred?

   [1 mark]

   • AGenetic drift
   • BThe founder effect
   • CSpeciation
   • DStabilising selection

   (c)

   Explain how disruptive selection changes the frequencies of the alleles for beak depth in the finch population.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Northern elephant seals were hunted to about 20 individuals in the 1890s. Their numbers have since recovered to over 100 000, but genetic analysis shows that they have much less genetic variation than southern elephant seals, which were not hunted so severely. A small group of northern elephant seals later colonised a new island.

   (a)

   Explain how the hunting of the northern elephant seals reduced their genetic variation.

   [3 marks]

   (b)

   Explain why the allele frequencies in the seals that colonised the new island may differ from those in the original population, and why genetic drift has a greater effect in small populations.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Phenylketonuria (PKU) is a genetic condition caused by a recessive allele. It occurs in about 1 in 10 000 births in a large human population in which random mating occurs and no selection, mutation or migration affects the allele frequencies. Assume that the population is in Hardy-Weinberg equilibrium.

   (a)

   Use the Hardy-Weinberg equation to calculate the frequency of the recessive allele, the frequency of the dominant allele, the frequency of carriers, and the number of carriers expected in a population of 500 000 people.

   [6 marks]

   (b)

   Evaluate the use of the Hardy-Weinberg equation to monitor changes in allele frequencies in a population such as this one.

   [6 marks]

   Total for question 4: 12 marks

End of questions