Discrete or indiscrete? Redefining the colour polymorphism of the land snail Cepaea nemoralis

Abstract

Biologists have long tried to describe and name the different phenotypes that make up the exuberant colour polymorphism of the land snail Cepaea nemoralis. Traditionally, the view is that the ground colour is one of a few major colour classes, either yellow, pink or brown, but in practise it is frequently difficult to distinguish the colours, and consistently define different shades of the same colour. To understand whether colour variation is continuous, and to investigate how the variation may be perceived by an avian predator, we applied psychophysical models of colour vision to shell reflectance measures. The main finding is that both achromatic and chromatic variation are indiscrete, being continuously distributed over many perceptual units, with the major axis of chromatic variation representing differences in saturation, or purity of colour. Nonetheless, clustering analysis based on the density of the distribution revealed three groups, roughly corresponding to human-perceived yellow, pink and brown shells. There is also large-scale geographic variation between these morphs across Europe, and some covariance between shell colour and banding patterns. Although further studies are necessary to understand the evolutionary origins and impact of natural selective upon this variation, the observation of continuous variation in colour is intriguing, given that the underlying supergene that determines colour should prevent phenotypes from \"dissolving\" into continuous trait distributions.