1. 1
   Cystic fibrosis is caused by a recessive allele of the CFTR gene; only homozygous recessive people are affected. In a large mainland population X, about 1 in 2500 babies is born with cystic fibrosis. In population Y, which lives on a remote island and has been geographically isolated from X for many centuries, about 1 in 10 000 babies is born with the condition. Assume both populations are in Hardy–Weinberg equilibrium for this gene.

   (a)

   What is the frequency of the cystic fibrosis allele (q) in population X?

   [1 mark]

   • A0.0004
   • B0.02
   • C0.04
   • D0.2

   (b)

   Which is the best explanation for the difference in allele frequency between populations X and Y?

   [1 mark]

   • AThe equation p + q = 1 does not apply to population Y
   • BMutations in the CFTR gene can only occur in population X
   • CGeographical isolation prevents gene flow, so allele frequencies in the two gene pools have changed independently
   • DHeterozygous carriers in population Y are affected by cystic fibrosis

   (c)

   Calculate the frequency of carriers of the cystic fibrosis allele in population Y.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Wild cabbage (Brassica oleracea) grows on coastal cliffs in Europe. Over many centuries, farmers kept seed only from the plants with the features they wanted. Choosing plants with large terminal buds produced cabbage, choosing enlarged flower clusters produced broccoli and cauliflower, and choosing large side buds produced Brussels sprouts. In recent decades, a weed growing in fields of these crops has been sprayed with the same herbicide every year. The proportion of weed plants that survive spraying has risen from under 1% to over 60%, although nobody chose to breed resistant weeds.

   (a)

   How is the development of broccoli from wild cabbage best described?

   [1 mark]

   • AArtificial selection
   • BNatural selection by an abiotic factor
   • CSexual selection
   • DInheritance of acquired characteristics

   (b)

   Which type of selection has herbicide spraying exerted on the weed population?

   [1 mark]

   • AArtificial selection
   • BStabilizing selection
   • CDisruptive selection
   • DDirectional selection

   (c)

   Explain why the evolution of herbicide resistance in the weed is an example of natural rather than artificial selection.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a moth species, a wing-pattern gene has two alleles, B and b, and all three genotypes can be recognised from the wing pattern: BB moths are pale, bb moths are dark and Bb moths are intermediate. A sample of 1000 adult moths from a large population contained 640 BB, 120 Bb and 240 bb moths. The moths rest by day on tree trunks in a woodland where some trunks are covered in pale lichen and others are dark and bare; intermediate moths do not closely match either background. In a separate small study, females appeared to prefer males with the same wing pattern as themselves.

   (a)

   Calculate the allele frequencies and the number of heterozygous moths expected in the sample if the population were in Hardy–Weinberg equilibrium.

   [3 marks]

   (b)

   Evaluate possible reasons why the observed genotype frequencies differ from those predicted by the Hardy–Weinberg equation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the Aripo River in Trinidad, guppies (Poecilia reticulata) living below a waterfall share the stream with the pike cichlid Crenicichla alta, a strong predator of adult guppies. Above the waterfall there were no guppies, only the weak predator Rivulus hartii. In 1976 John Endler moved 200 guppies from below the waterfall into the guppy-free stream above it. About two years later, after roughly 15 generations, males in the transferred population had more spots, and larger and brighter spots, than males in the source population below the waterfall, and they resembled males from natural low-predation streams. The waterfall prevents guppies moving upstream. When males from the transferred population were bred in aquaria, their sons had similar colour patterns.

   (a)

   Explain the change in male colour pattern in the transferred population.

   [6 marks]

   (b)

   Discuss how this experiment illustrates the neo-Darwinian explanation of evolution, referring to the gene pool and to types of selection.

   [6 marks]

   Total for question 4: 12 marks

End of questions