Data from: Change and stability in a steep morph-frequency cline in the snail Cepaea nemoralis (L.) over 43 years.

Populations of the polymorphic land snail Cepaea nemoralis (L.) from
Deepdale, Derbyshire, UK, sampled in 1965-67, showed a pattern of area
effects, with steep clines among groups of populations differing in shell
colour and banding morph frequencies. In 2010 most of these populations
were resampled. In particular a continuous transect made in 1967 of 42
18.34m x 18.34m quadrats across a steep cline in several morph frequencies
was completely resampled. In the dale as a whole, yellow shells had
increased in frequency. In the transect the frequencies of banding morphs
showed no significant changes, but colour morphs showed some changes. Pink
shells had increased in frequency in a section in which scrub had
developed, and brown shells had increased in frequency in the area in
which they had originally been at the highest frequency. In each case, the
selection coefficients were of the order of 4%. Yellow had increased
elsewhere. Nevertheless, both in the dale as a whole and in the transect,
the pattern of geographical change in morph frequencies had remained
essentially the same. Estimates of migration based on previous studies of
marked snails and on modelling of the effect of drift and migration
suggest that whether the cline is a product of differential selection or
of the gradual merging of previously separate founding populations, it has
been in existence for a long time, and that migration occurs over greater
distances than estimated from direct observation on marked snails. While
we can demonstrate that selection has occurred, the origin and maintenance
of the cline and others like it remain in doubt; the development and
maintenance of polymorphism in this species may require consideration of
several processes operating on different time scales.