• centripetal ;
  • diversifying ;
  • gametic excess ;
  • linkage ;
  • visible polymorphism

The snail Cepaea nemoralis (L.) is usually polymorphic for linked loci conferring pink or yellow shell ground colour and banding or lack of bands. An unlinked locus, mid-banded, modifies the appearance of banded shells. These characters have a very wide range of frequencies across the species range and even within limited areas. In Britain, frequencies differ between populations from shaded woodland habitats and those from closely adjacent open habitats. Using data from the Evolution MegaLab Project, it is shown that the greater frequency of pink unbanded in shaded habitats is associated with a greater excess of the favoured allele combinations, measured as linkage disequilibrium. Examination of the relationship of disequilibrium to allele frequency in samples from the full geographical range and all habitats and climates shows that, in general, the allele combinations at high frequencies are in gametic excess. This suggests that selection tends to diversify rather than move frequencies towards stable equilibria. The result is important in relation to the as yet unresolved question how the polymorphism is maintained. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●●, ●●–●●.