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

• Published in: bioRxiv
• Main Authors: Davison, Angus, Jackson, Hannah J, Murphy, Ellis W, Reader, Tom
• Format: Paper
• Language: English
• Published: Cold Spring Harbor Cold Spring Harbor Laboratory Press 07.08.2018
• Subjects: Color; Color vision; Gene polymorphism; Phenotypes; Psychophysics; Variation
• DOI: 10.1101/383042

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. In practise, it is frequently difficult to distinguish the colours, and consistently define different shades of the same colour. We therefore applied psychophysical models of colour vision to shell reflectance measures, so as to understand how the variation is perceived by an avian predator. 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 did reveal 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. 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 particularly intriguing, given that the underlying supergene that determines colour should prevent phenotypes from "dissolving" into continuous trait distributions.