1. 1
   Horses (Equus caballus) have 64 chromosomes in their body cells and donkeys (Equus asinus) have 62. When the two are kept together by breeders, a male donkey and a female horse can mate to produce a mule, which has 63 chromosomes and is almost always sterile. Where the ranges of wild horses and wild asses overlapped in the past, hybrids were very rarely formed, because the two species differ in their courtship and mating behaviour.

   (a)

   Which statement best explains why mules are almost always sterile?

   [1 mark]

   • AMule chromosomes cannot form homologous pairs in meiosis, so viable gametes are very rarely produced
   • BAn odd number of chromosomes prevents mitosis, so the mule's gonads do not grow
   • CMules inherit an allele for sterility from the donkey parent
   • DMules are triploid, with three sets of chromosomes

   (b)

   Horses and donkeys can mate and produce offspring, yet they are classified as separate species. Which statement best justifies this?

   [1 mark]

   • AThey look different from each other
   • BThey cannot produce any offspring together
   • CTheir hybrid offspring are infertile, so alleles cannot pass between the two gene pools
   • DThey were domesticated by humans at different times

   (c)

   Explain how differences in courtship behaviour and hybrid sterility together prevent the mixing of alleles between horse and donkey populations.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The forelimbs of a human, a mole, a dolphin and a bat all contain the same set of bones: one upper bone (humerus), two lower bones (radius and ulna), a group of wrist bones (carpals), palm bones (metacarpals) and finger bones (phalanges) arranged in five digits. The proportions differ: the mole's forelimb is short and broad for digging, the dolphin's forms a stiff flipper, and in the bat the metacarpals and phalanges are very long and support a membrane of skin used for flight. Butterflies also fly, using wings made of chitin with no bones.

   (a)

   Which term describes the forelimbs of the four mammals?

   [1 mark]

   • AAnalogous structures
   • BHomologous structures
   • CVestigial structures
   • DProducts of convergent evolution

   (b)

   Which pair of structures is analogous?

   [1 mark]

   • AHuman arm and bat wing
   • BDolphin flipper and mole forelimb
   • CHuman hand and mole forelimb
   • DBat wing and butterfly wing

   (c)

   Explain why the pentadactyl limb is regarded as evidence for evolution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Many species in the knotweed genus Persicaria have arisen by hybridization and polyploidy. In a study of three Persicaria species growing together in the same wetland (here called X, Y and Z), species X had 22 chromosomes in its body cells and species Y had 20. Species Z had 42 chromosomes, and analysis showed that these were the full chromosome sets of both X and Y. Hybrids between X and Y, with 21 chromosomes, grew well but produced almost no viable seeds. Plants of Z were fully fertile and could self-pollinate.

   (a)

   Explain why the hybrid of X and Y is sterile, and how doubling of its chromosome number could have produced the fertile species Z.

   [3 marks]

   (b)

   Deduce why Z is a separate species from X and Y, and discuss why its formation is described as abrupt, sympatric speciation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Lake Victoria in East Africa held more than 500 species of cichlid fish, most of them found nowhere else. Evidence suggests that the lake was almost dry about 15 000 years ago and that most of these species evolved within the lake since it refilled, from a small number of ancestral species. The species differ in jaw and tooth shape and feed on algae scraped from rocks, snails, insect larvae, plankton or other fish. Many closely related species live in the same parts of the lake. Females choose mates by the colour of the males, and closely related species often differ mainly in male colour. Some species are confined to separate rocky outcrops separated by stretches of open sand that they do not cross.

   (a)

   Using the cichlids as an example, explain the roles of reproductive isolation and differential selection in speciation.

   [6 marks]

   (b)

   Discuss whether speciation of the Lake Victoria cichlids is more likely to have been allopatric or sympatric, and explain how adaptive radiation has allowed so many species to coexist.

   [6 marks]

   Total for question 4: 12 marks

End of questions