Qualitative and quantitative methods show stability in patterns of Cepaea nemoralis shell polymorphism in the Pyrenees over five decades.

Daniel Ramos-Gonzalez,Angus Davison

doi:10.1002/ece3.7443

Abstract

Over the past century, the study of animal color has been critical in establishing some of the founding principles of biology, especially in genetics and evolution. In this regard, one of the emerging strengths of working with the land snail genus Cepaea is that historical collections can be compared against modern‐day samples, for instance, to understand the impact of changing climate and habitat upon shell morph frequencies. However, one potential limitation is that prior studies scored shell ground color by eye into three discrete colours yellow, pink, or brown. This incurs both potential error and bias in comparative surveys. In this study, we therefore aimed to use a quantitative method to score shell color and evaluated it by comparing patterns of C. nemoralis shell color polymorphism in the Pyrenees, using both methods on present‐day samples, and against historical data gathered in the 1960s using the traditional method. The main finding was that while quantitative measures of shell color reduced the possibility of error and standardized the procedure, the same altitudinal trends were recovered, irrespective of the method. The results also showed that there was a general stability in the local shell patterns over five decades, including altitudinal clines, with just some exceptions. Therefore, although subject to potential error human scoring of snail color data remains valuable, especially if persons have appropriate training. In comparison, while there are benefits in taking quantitative measures of color in the laboratory, there are also several practical disadvantages, mainly in terms of throughput and accessibility. In the future, we anticipate that genomic methods may be used to understand the potential role of selection in maintaining shell morph clines. In addition, photographs generated by citizen scientists conducting field surveys may be used with deep learning‐based methods to survey color patterns.

Highlights

The study of animal color has been critical in establishing some of the founding principles of biology, especially in genetics and evolution
Inherited variation in animal color has been used to understand the relative roles that natural selection and random genetic drift have to play in the establishment and maintenance of color polymorphism
The main finding was that while quantitative measures of shell color reduced the possibility of error, and standardized the procedure, the same altitudinal trends were recovered, irrespective of the method

Summary

| INTRODUCTION

The study of animal color has been critical in establishing some of the founding principles of biology, especially in genetics and evolution. Yellow and midbanded morphs had increased in woodland, whereas unbanded and midbanded increased in hedgerow habitats (Cook, 2014) In comparison with these large surveys, the majority of comparative studies have been at a more local scale. Modern comparative surveys have tended to find an increase in yellow and midbanded shells (as above) (Cameron et al, 2013; Ożgo et al, 2017; Ożgo & Schilthuizen, 2012; Silvertown et al, 2011), but with exceptions (Cameron & Cook, 2012; Cook et al, 1999; Cowie & Jones, 1998), depending upon the precise scale of comparison. | 3 color polymorphism using both methods on present-day samples, and against historical data gathered using the traditional method To achieve this aim, the Central Pyrenees were used as an exemplar location, because they were intensively surveyed during the 1960s and 70s (Figures 1 and 2), sometimes showing sharp discontinuities of frequencies of morphs within and between valleys. As C. nemoralis in the Pyrenees is polymorphic for other characters, we scored the lip color, as either pale (usually white) or any other color (usually black or dark brown), and measured the shell height (H) and width (W) using a Vernier calliper with 0.05 mm precision, calculating the shape as H/W

| MATERIALS AND METHODS

Findings

| DISCUSSION