Colour and pattern disequilibrium in Cepaea nemoralis on a northern European transect

Disequilibrium has been estimated for alleles at pairs of loci controlling shell colour and banding in populations of Cepaea nemoralis from northern Europe. There is strong overall positive disequilibrium between pink/yellow colour and unbanded/banded (pink unbanded in excess) but not for colour and the band‐modifying loci mid‐banded and trifasciate. When the colour/banding data are divided into four equally sized sections (quadrants) representing, respectively, high and low yellow and high and low unbanded frequencies, there is significant interaction in disequilibrium between quadrants, with high positive disequilibrium contributed by populations in which pink and unbanded are at high frequency. We interpret this as indicating diversifying, rather than stabilizing, selection. Simulations show how the overall value and quadrant differences may have built up. The brown allele at the colour locus is known to be associated with much higher unbanded frequencies than the other two colours, owing to some intrinsic incompatibility between banding and ground colour. It is suggested that the same may apply, although to a lesser extent, with respect to pink and unbanded relative to yellow.