By Sam Careless

Banded snails display a variety of brilliant colours. In most cases the banded snail will have five brown or black bands running parallel to the whorls of the shell.

Their various shell colours and patterns camouflage them against different backgrounds within their habitat , such as: grass, tree bark and foliage.

These snails are widespread in Europe where they

are regularly eaten and have more recently been

introduced to North America. They can be found

in woodlands, copses, hedges and open country.

Within the varying habitats, there are many different predators that prey on the banded snail. Some of these include rats, voles, hedgehogs, field mice and even rabbits. Thrushes consider the banded snail one of their favourites, holding the shell in their beak and hammering them against a stone to get at their prey.

To protect themselves from their main predator: thrushes , the snails use their coloured shells to camouflage themselves within the environment.

However not all coloured shells are completely successful at blending in with the environment, and as a result certain coloured snails survive longer than others. E.g. In the woodland habitat, generally the browner the shell, the more camouflaged it is from its predators.

As the brown snail shells are less likely to be eaten by predators, they are more likely to breed and reproduce compared with other coloured shells.

The S.D graph here shows the frequency of snails found in a woodland area, compared with their colour of shell.

The x axis values correspond with the following in ascending order of colour darkness: 0 – very light (white), 4 – fairly dark (brown) , 8 – very dark (black)

• The mean frequency corresponds for the brown coloured shell , which shows in this environment the brown banded shell is the mean selected allele for the snail shell colour.

Previously, the mean allele for the shell colour may of been different:

• Here the mean distribution has shifted left, showing that previously the mean selected allele for shell colour was between 2-3 – a much lighter colour.

• However, a light coloured shell would provide less camouflage than a dark one, so predators such as thrushes would be more likely to catch them. The brown-shell allele would have been present in some snails, though only a small proportion as shown in the graph.

• With the more hidden colour, brown-banded snails are less likely to be eaten by predators, they are more likely to breed and reproduce compared with other, lighter coloured shells. This would consequently increase the proportion of the brown-shell allele within the population, eventually shifting the distribution graph to the right.

The brown-shell allele exists within 2.1% of the snail population.

Conclusion As the proportion of the brown-shell allele greatly increases, the allele

becomes the “selected” phenotype.

Gradually the population of banded snails will show the brown-shell phenotype and result in the current distribution:

The brown-shell allele is the mean shell colour of the snail population.

The brown-shell allele exists within 2.1% of the snail population.

Previous Distribution Current Distribution