Spatial structure of shell polychromatism in populations of Cepaea nemoralis: new techniques for an old debate

A conspicuous shell polychromatism is observed in colonies of Cepaea nemoralis from western France (Brittany). The present study is intended to search for a spatial structure of shell features at this scale and to infer evolutionary processes from the observed patterns. We used a database of morph frequencies (six composite phenotypes were retained) measured on 213 samples regularly distributed on the whole studied area. Data analysis was based on two distinct multivariate methods leading to the following steps: (i) to search for a structure without reference to environmental conditions with a method (global principal components analysis: GPCA) which takes into account the spatial information by means of a neighbouring relationship between sampling points (Delaunay triangulation); (ii) to test the structuring power of environmental conditions by means of two explanatory factors (distance from the sea, altitude) involved in a redundancy analysis (RDA); (iii) to search for a spatial structure using residuals of the previous analysis, ie, after removing effects of environmental conditions. Global covariance accounted for 26.4% of the total variance, leading to a highly significant autocorrelation for each phenotype (step 1). Geographical mapping of factorial scores resulting from global analysis showed a well structured littoral zone and a strong southern-northern inland differentiation. Sixteen percent of the total variance was expressed in RDA but all morphs were not equally concerned. After removing environmental effects, a significant spatial structure still remains but was essentially caused by random processes. We argue for the importance of these last phenomena.