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Date	May 2018	Marks available	3	Reference code	18M.2.SL.TZ1.1
Level	Standard level	Paper	Paper 2	Time zone	Time zone 1
Command term	Explain	Question number	1	Adapted from	N/A

Question

The land snail Cepaea nemoralis is very common in North America and in Europe.
The base colour of its shell varies between brown, pink and yellow, and also in its intensity. Some shells are unbanded, but most show one to five bands of different width on top of the shell base colour.

In the early 1950s, scientists studied the proportion of colours and banding of C. nemoralis in woods and fields near Oxford, UK, which differed in the type of plants and background colour. Each data point on the graph represents the percentage of yellow base colour shells and unbanded shells in a sample from either one type of wood or field, although other snail colours were present.

The population of C. nemoralis has been studied for many years in open fields in a similar area. In the graph, each data point represents the percentage of adults of a given base colour plotted against the percentage of juveniles of the same base colour collected each year.

Determine the maximum percentage of yellow base colour shells found in woods.

[1]

Suggest either one possible advantage or one disadvantage of having a banded shell, stating whether it is an advantage or disadvantage.

[1]

Using the data in the graph, distinguish between the distribution of C. nemoralis shells in woods and fields.

[2]

Deduce from the data in the graph which shell base colours are on average most and least frequent among adult snails.

Most frequent:

Least frequent:

[2]

Discuss whether there is evidence in the data that colour plays a role in the survival of the snails.

[3]

Using the theory of natural selection, explain the differences shown in the graph between the three colours of snail.

[3]

Markscheme

78 (%)

Accept answers ranging between 77 and 80 %.

advantage: camouflage
OR
disadvantage: visibility

a. more yellow in fields (than in woods) / vice versa

b. more unbanded in woods (than in fields) / vice versa

c. more overlap within banding than within yellow colour
OR
yellow colour range greater than banding range

d. very little overlap between fields and woods / some outliers

Do not accept answers with only numerical data.

a. brown most frequent

b. pink least frequent

Evidence that colour plays a role:
a. few yellow adults (relative to juveniles) means that yellow juveniles do not survive into adulthood

b. frequent brown adults (relative to juveniles) means brown juveniles survive well into adulthood

Evidence that colour does not play a role:
c. similar numbers of adult and juvenile pink individuals means pink colour does not play a role

d. all three colours show wide variation/considerable overlap therefore evidence is not strong

a. natural selection requires that snails become adults/live to reproduce their variations/undergo differential predation OWTTE

b. higher adult frequency of brown shows selection

c. but results for pink do not show selection

d. more brown juveniles survive into adulthood showing that brown is selected for/vice versa against yellow

e. not enough alone to support theory of natural selection but may be added evidence to similar observations in other organisms OWTTE

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Core » Topic 5: Evolution and biodiversity » 5.2 Natural selection

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