Exam-style Questions: Evolution

1. Cepaea nemoralis is a common British snail which is found in a variety of habitats. The shells of this species of snail vary in the pattern of dark bands found on the surface. The drawings show a banded snail and an unbanded snail.

Song thrushes feed on these snails. A bird finds a snail and takes it to a suitable stone known as an anvil. It hits the snail shell against the stone, breaks it open and eats the soft parts. The remains of the shell can be found on the ground near the thrush's anvil stone.

In one investigation, two areas of woodland floor were cleared of all snails and equal numbers of banded and unbanded snails were then introduced. Over the next two weeks the snail shells found around the anvil stones in one area were compared with the shells of living snails found in a control area where there were no thrushes. The results are shown in the table.

Number of unbanded snails

Number of banded snails

Snail shells found around anvil stones

153

264

Shells of living snails found in control area

204

217

a) Describe and explain the effect of predation by thrushes on the snails in this investigation.

b) The presence of bands on the shells of C. nemoralis is controlled by a single gene with two alleles. Explain how natural selection might account for different proportions of these alleles.

Explain how natural selection might account for different proportions of these alleles in different habitats.

c) In order to collect the sample of living snails in the control area, quadrats were placed at random in the area and all the snails found in the quadrat were collected.

Explain why it is necessary to:

(i) Place the quadrats at random;

(ii) Ensure that every snail found within the quadrat was collected.

d) In a seperate investigation, the population of C. nemoralis was estimated using the mark-release-recapture method.

(i) Giving a reason for your answer, describe where on the shell you would mark the snails.

(ii) Explain why the results would be more accurate if there was only a short time between releasing the marked snails and catching the second sample.

(Marks available: 8)

Answer

Answer outline and marking scheme for question: 1

Give yourself marks for mentioning any of the points below:

a) Take higher proportion of banded snails;
Unbanded better camouflaged/less conspicuous
to predators.

(2 marks)

b) Different proportions of banded and Unbanded snails survive in different habitats;
Able to breed and pass on selected alleles/genes.

(2 marks)

c) (i) Avoid bias/get representative sample/enable
statistical test to be applied

(ii) Otherwise more conspicuous snails might be
collected.

(2 marks)

d) (i) Mark undersurface/inside lip otherwise they will be more conspicuous/likely to be predated/harm the animal.

(ii) They would be removed from the population by the thrushes/snails dying/immigration/emigration/breeding.

(2 marks)

(Marks available: 8)

2. Write an essay on the causes and nature of the variation on which natural selection depends.

(Marks available: 25)

Answer

Answer outline and marking scheme for question: 2

You can give yourself up to 2 marks for discussing any of the following issues up to a maximum of 16:

Nature of variation:

  • Clear explanation of continuous variation, including the idea of frequency distribution.
  • Explanation of the roles of genetics .and. the environment in producing variation;
  • Continuous variation -accounted for in terms of polygenic characters.

Causes of variation:

  • Reshuffling of genes via sexual reproduction e.g. meiosis, independent assortment, crossing over, random fertilisation - (any two of these);
  • Mutation as the random/main cause of (persistent) variation affecting natural selection;
  • Types of mutation explained (chromosomal and gene) OR given in terms of heritability i.e. germ cf. somatic cell mutation;
  • Cause of mutation (high energy radiation/ particles OR named chemical mutagen);
  • Gene mutation as the result ofa change in (sequence of bases) in DNA which may result in change in the (amino acid sequence of a) polypeptide;
  • Such a change in polypeptide structure may result in change in way protein functions;

Variation in relation to Natural Selection:

  • Explain how (any 2 of) predation, disease, competition give differential survival/reproduction;
  • Explain that organisms with selective advantages are more likely to survive, reproduce and pass on their genes to the next generation;
  • Explain how the process ofnatural selection may result in changes in the allele and phenotype frequency and may lead to the fonnation of new species;
  • Quote a specific example to explain how variation is acted upon via natural selection to produce changes within species.

Breadth (max 3 marks)

Does the answer appropriately:

  • Distinguish continuous/discontinuous variation.
  • Consider variation in relation to natural selection.
  • Show a wide view of causes of variation.

Depth (max 3 marks)

Does the answer appropriately explain:

  • Effects of genetics or environment on variation.
  • That genetic variation/mutation is necessary for natural selection.
  • Consider variation in terms of DNA.

(Marks available: 25)

3. Write an essay on:

The interactions between organisms of the same and different species

(Marks available: 25)

Answer

Answer outline and marking scheme for question: 3

You can give yourself up to 2 marks for discussing any of the following issues in A, B, C and D up to a maximum of 16:

A. General:

1. Explain that interactions may (profoundly) influence the distribution OR size of populations;

2. Make clear that an ecosystem can only support a certain size of population, which varies according to limiting factors;

3. Gives factors affecting population size (3 or more of) food supply space, light, predation and disease;

4. Explains niche - position or status of organism within community resulting from its adaptations, physiology and behaviour, including its energy source, period of activity etc.

B. Types of interaction:

1. Explains predation, giving nature and example, and balance between predator/prey populations (relative population sizes, that are rarely stable, and periodically fluctuate);

2. Explains principle of competition - i.e. when niches overlap - interactions between populations in which each adversely affects the other (in e.g. food, nutrients, space, light, or other common need);

3. General explanation of (1 interspecific) association - parasitism, commensalisms, mutualism (symbiosis);

4. Pollination/dispersal - discussion of either, with a suitable example;

5. Suitably explains coloration -cryptic or disruptive example;

6. Suitably explains mimicry and warning coloration, with example of Batesian, Mullerian, automimicry.

C. Inter and intraspecific interactions:

1. Detailed example of one interspecific association e.g. parasitism or mutualism;

2. Suitable example of intraspecific association e.g. social organisation in insects, or in mammals;

3. Clear explanation of interspecific competition with suitable example;

4. Clear explanation of intraspecific competition with suitable example.

D. Possible additional areas:

1. Suitably thorough treatment of human influence, with e.g. explanation of biological control;

2. Suitable treatment of competition in relation to natural selection - competitive exclusion, specialization through competition, competition and evolution.

Breadth (3 marks)

Does the answer appropriately:

  • deal with a full range of factors (in A3)?
  • give examples for both animals and plants?
  • discuss at least 4 types of interaction?

Selection of appropriate material Depth) (3 marks)

Does the answer appropriately:

  • make clear the concept of a niche?
  • distinguish interspecific/intraspecific interaction?
  • distinguish density independent and dependent factors?

In the real exam you will also be awarded marks for quality of language (in this case, there were 3 marks available). It may be difficult for you to judge your own text, however you may be able to get someone else to give you an opinion.

(Marks available: 25)