Populations and evolutionAQA A-Level Biology: Topic test

1. 1
   Phenylketonuria (PKU) is a recessive condition caused by the allele $n$. The normal allele is $N$. In one population, 1 in 10 000 babies is born with PKU. Assume that the population is in Hardy–Weinberg equilibrium.

   (a)

   What is the frequency of the allele $n$ in this population?

   [1 mark]

   • A0.0001
   • B0.1
   • C0.01
   • D0.99

   (b)

   What is the expected frequency of carriers (heterozygotes) in this population?

   [1 mark]

   • A0.0198
   • B0.0001
   • C0.0099
   • D0.9801

   (c)

   Calculate the number of carriers expected in a population of 250 000 people. Show your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Rock pocket mice live on both pale sandy rock and dark volcanic lava. On the dark lava, owls catch more pale-furred mice than dark-furred mice. Fur colour is controlled by one gene and the dark allele is dominant. Over many generations the frequency of the dark allele in the population living on the lava has increased greatly.

   (a)

   Which factor is the selection pressure acting on the mice on the lava?

   [1 mark]

   • AMutation
   • BPredation by owls
   • CGenetic drift
   • DCompetition for food

   (b)

   Which type of selection has acted on fur colour in the mice on the lava?

   [1 mark]

   • AStabilising selection
   • BDisruptive selection
   • CArtificial selection
   • DDirectional selection

   (c)

   Explain how the frequency of the dark allele increased in the population on the lava.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A biologist studies one gene, with alleles $A$ and $a$, that has no effect on survival or reproduction in a species of tree frog. Two populations began with the frequency of each allele at 0.50. After 20 generations the frequency of $a$ was 0.90 in population X, which has about 25 breeding adults, and 0.52 in population Y, which has about 2500 breeding adults.

   (a)

   Calculate how many times greater the change in the frequency of $a$ was in population X than in population Y, and explain why the change is greater in X.

   [3 marks]

   (b)

   The biologist concludes that natural selection is unlikely to have caused the change in population X. Justify this conclusion, predict what will happen to the genetic variation in X, and name the two forces that affect genetic variation in populations.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A crater lake with no inflow or outflow was colonised once, thousands of years ago, by a single species of cichlid fish. Today the lake contains two species. One lives in the surface waters and feeds on insects. The other lives at a depth of 20 m and feeds on snails. Females of each species choose mates by colour, and the light at each depth favours a different colour vision allele. The two species do not interbreed in the lake.

   (a)

   Explain how the two species could have arisen from the original single population while living in the same lake.

   [6 marks]

   (b)

   In the surface species, 36% of the fish are homozygous recessive for a gene that affects eye pigment. Assume that the Hardy–Weinberg principle applies. Calculate the frequency of the dominant allele and the percentage of heterozygous fish, state two conditions needed for the principle to apply, and explain why strong selection on this gene would mean that it did not apply.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Two populations of a butterfly species lived on either side of a mountain range that slowly rose over two million years. The western population breeds in spring and the eastern population breeds in late summer. Erosion has since lowered the mountains, and the two populations now live in the same valleys but do not interbreed.

   (a)

   Which term describes the way in which the two new species formed?

   [1 mark]

   • ASympatric speciation
   • BStabilising selection
   • CGenetic drift
   • DAllopatric speciation

   (b)

   Which feature now prevents gene flow between the two populations?

   [1 mark]

   • AThe mountain range
   • BDifferent breeding seasons
   • CA difference in chromosome number
   • DCompetition for the same food

   (c)

   Explain why the separation of the populations by the mountain range could lead to the formation of two species.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A botanist takes cuttings from a single wildflower plant and plants them in two plots, one with rich soil and one with poor soil. The cuttings in the rich soil grow to 90 cm and those in the poor soil grow to 45 cm. She also collects seeds from the same wildflower and grows them all in the rich soil. The seedlings grow to different heights, between 60 cm and 100 cm.

   (a)

   Which processes produce the genetic differences between the seedlings?

   [1 mark]

   • AMeiosis and random fertilisation
   • BMitosis and vegetative growth
   • CDifferences in soil nutrients
   • DNatural selection

   (b)

   Which statement explains why the cuttings in the poor soil are shorter than those in the rich soil?

   [1 mark]

   • AThey have different alleles from the cuttings in the rich soil.
   • BA mutation occurred in the cuttings in the poor soil.
   • CThe environment affects the expression of the genotype.
   • DNatural selection has favoured the shorter plants.

   (c)

   Explain why only the genetic differences between plants, and not the differences caused by the soil, can lead to evolution.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   In a population of 500 butterflies, wing colour is controlled by one gene with a dominant allele $O$ (orange) and a recessive allele $o$ (yellow). Twenty of the butterflies are yellow. Assume that the population is in Hardy–Weinberg equilibrium.

   (a)

   Calculate the frequency of the allele $o$ and the frequency of the allele $O$. Show your working.

   [3 marks]

   (b)

   Calculate the number of heterozygous butterflies and the number of homozygous orange butterflies expected in the population. A later count finds 140 heterozygous butterflies. Suggest one reason why this differs from the expected number.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   The cheetah population is thought to have been reduced to a very small number of individuals about 10 000 years ago and now numbers several thousand. Studies show that cheetahs have very little genetic variation: at most gene loci almost every individual carries the same allele. Cheetahs are also highly susceptible to some infectious diseases.

   (a)

   Explain how a reduction to a very small population can cause low genetic variation that persists after the population has recovered in size.

   [6 marks]

   (b)

   Compare the effects of natural selection and genetic drift on allele frequencies in a cheetah population when a new infectious disease appears.

   [6 marks]

   Total for question 8: 12 marks

End of questions