1. 1
   In a laboratory study, researchers cut the tails off adult mice before allowing them to breed, and repeated this in every generation for 20 generations. Every mouse born in every generation had a tail of normal length. In the 12th generation, a single mouse with a pale coat was born to two normally coloured parents. The colony's breeding records showed no pale mice in its previous 60 years, and DNA sequencing showed that the pale mouse carried a base substitution in a pigment gene that neither parent carried. When the pale mouse was bred, some of its offspring also had pale coats.

   (a)

   Which idea do the results of the tail-cutting experiment refute?

   [1 mark]

   • AMutation generates new alleles
   • BSexual reproduction generates new combinations of alleles
   • CNatural selection requires variation between individuals
   • DCharacteristics acquired during an individual's life are passed on to its offspring

   (b)

   Which is the best explanation for the appearance of the pale mouse?

   [1 mark]

   • ACrossing over in the pale mouse's parents created a new allele
   • BThe laboratory environment made the mouse pale and this change was then inherited
   • CA mutation in a gamete-forming cell of one parent produced a new allele
   • DRandom fertilisation brought together two copies of an allele already carried by both parents

   (c)

   Explain why cutting off the tails had no effect on the tail length of the offspring.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   John Endler studied guppies (Poecilia reticulata), small fish from streams in Trinidad. Males carry coloured spots, and females prefer to mate with males that have more and brighter spots. In a greenhouse, Endler stocked a set of identical gravel-bottomed ponds with guppies from a mixed source; at the start the mean number of spots per male was 12.5 in every pond. Some ponds had no predators, some had Rivulus hartii (a weak predator that rarely eats adult guppies) and some had the pike cichlid Crenicichla alta (a strong predator of adult guppies). After about 20 months (roughly ten generations), the mean number of spots per male was 13.6 in ponds with no predators, 13.9 in ponds with Rivulus and 9.1 in ponds with the pike cichlid.

   (a)

   Which selection pressure most likely caused the increase in mean spot number in the ponds with no predators?

   [1 mark]

   • AFemales choosing to mate with males that have more spots
   • BPredators eating the males with the fewest spots
   • CCompetition between females for food
   • DMales gaining spots by using their colour patterns during display

   (b)

   Which conclusion is best supported by the results?

   [1 mark]

   • ARivulus causes mutations that increase the number of spots
   • BStrong predation selects against conspicuous males more strongly than female choice selects for them
   • CSpot number is not a heritable trait
   • DSexual selection stops acting when predators are present

   (c)

   Explain the value of including ponds with no predators in this experiment.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   On the Galápagos island Daphne Major, medium ground finches (Geospiza fortis) feed mainly on seeds. Beak depth varies between individuals, and in earlier breeding studies the beak depth of offspring closely matched the mean of their parents. In 1977 a severe drought meant that plants produced very few seeds. Small, soft seeds were eaten first, leaving mostly the large, hard seeds of Tribulus cistoides, which only deep-beaked birds could crack. The finch population fell from about 1200 to about 180. Mean beak depth was 9.4 mm in the population before the drought and 9.9 mm among the survivors. Birds that hatched in 1978, whose parents were drought survivors, had a mean beak depth of 9.8 mm.

   (a)

   Explain the difference in mean beak depth between the population before the drought and the survivors.

   [3 marks]

   (b)

   Evaluate the claim that the drought caused evolution in this finch population.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In 2017, a few days before two major hurricanes struck, researchers measured the lizard Anolis scriptus on two small Caribbean islands. Six weeks after the hurricanes they measured the survivors on the same islands. Relative to body size, survivors had toepads about 9% larger and forelimbs slightly longer, but hindlimbs shorter, than the lizards measured before the storms. In a wind-tunnel test, lizards gripped a perch with their front feet while their hindlimbs, held out from the body, caught the wind; lizards were blown off more easily if they had long hindlimbs. Females lay a single egg every one to two weeks through a long breeding season, and in normal years the number of lizards on each island is limited by the supply of insects. Long hindlimbs allow faster running on broad surfaces such as tree trunks and the ground.

   (a)

   Explain how the hurricanes acted as a selection pressure on this lizard population.

   [6 marks]

   (b)

   Discuss how overproduction of offspring and competition for resources are likely to affect selection on limb length in this population in the years after the hurricanes.

   [6 marks]

   Total for question 4: 12 marks

End of questions