Cline in
shell polymorphism in Cepaea nemoralis
Shell patterns in the common European land snail (Cepaea
nemoralis) are determined genetically. Studies
show that visual predators such as thrushes preferentially
consume the less cryptic snails in any environment. Dark,
un-banded snails are less common in bright grasslands.
However, adjacent population in similar environments vary
widely in handedness: notice sharp differences on either
side of the river at top. This is attributed to the snails'
notoriously low mobility, which allows local colonies to
remain separate.