The Brown-lipped or Grove snail, Cepaea nemoralis has received a lot of attention by evolutionary biologists for more than a century, due to their strikingly variable shell colour - what is called colour polymorphism. In the decades of the middle of the last century it was a very popular research organism. The shiny shell can be yellow, pink or brown. Over each of these background colours there can be no bands, one band or five bands, and the bands can also be fused and be of variable width. The snail above, which we found yesterday feeding on the fallen leaves on the garden path, is a yellow/one banded one. This polymorphism happens within the same population, but what puzzled biologists was the occurrence of sharp changes in the frequency of colour forms from one population to the next, and these differences seem to persist with time. This phenomenon was called "area effects". Many explanations have been proposed through the years to explain how the polymorphism is maintained and how area effects come to be, from differential predation (especially by song thrushes), adaptation to microhabitat, or other forms of selection to chance effects due to colonization after the glaciations, genetic linkage, dispersal between populations, etc. Many of these factors are not mutually exclusive and seem to have different importance depending on the population.
We found the shells below in the beach in Spurn Head a few years ago, all in a small area. They are a bit bleached by the sun, but you can see yellow and pink snails and three types of banding patterns.
Cain AJ, & Sheppard PM (1954). Natural Selection in Cepaea. Genetics, 39 (1), 89-116 PMID: 17247470
Davison, A., & Clarke, B. (2000). History or current selection? A molecular analysis of 'area effects' in the land snail Cepaea nemoralis Proceedings of the Royal Society B: Biological Sciences, 267 (1451), 1399-1405 DOI: 10.1098/rspb.2000.1156
Paul J. Mensink & Hugh A. L. Henry (2011). Rain event influence short-term feeding preferences in the snail Cepaea nemoralis Journal of Molluscan Studies. DOI: 10.1093/mollus/eyr011