The snail Cepaea nemoralis may have a yellow, pink or brown shell. Each colour shell may have up to five dark bands, or have no bands. Both shell colour and number of bands are genetically controlled. The snails are eaten by birds such as thrushes, which hunt by sight.

The following observations were made:

• Most snails living on a uniform background, such as short grass, have no bands.

• Most snails living on a green background, such as grass, are yellow.

• Most snails living on a non-uniform background, such as rough vegetation, have bands.

a. Suggest an explanation for these observations.

b. Predict the phenotype of snails living on a dark background of dead leaves.

c. Suggest what will happen, during the course of a year, to the frequencies of the different alleles controlling shell colour and banding in a snail population living in deciduous woodland. Deciduous trees shed their leaves in autumn. The background for the snails will be made up of dead leaves in the autumn and winter, and green vegetation in the spring and summer.