Polymorphic snails on varied backgrounds

The general matching of shell colours to background colours seen in terrestrial gastropods implies that, as in insects, visual predation is an important factor which has a long term directional effect. Polymorphism in snails is associated with background heterogeneity, but the causal relation of polymorphism to heterogeneity is not obvious. Predation could maintain polymorphism if predators are frequency dependent in their choice of prey. However, the appropriate predator behaviour does not depend directly on background heterogeneity. An indirect contribution could be that the heterogeneity serves to lower the signal:noise ratio during the predation process. Background heterogeneity could have a direct effect if the background provided specific elements mimicked by the morphs. The remaining diversity could aid the process by lowering the signal:noise ratio. Polymorphism could be maintained if there was frequency-dependent niche selection on the part of prey. Background diversity would then be directly involved. It is necessary that there should be independent control of numbers in the different niches. In warm conditions the niche selection could come about because darker morphs, which gain more radiant heat than paler ones, will move to more cryptic sites by seeking the shade. For morph frequences to approach equilibrium values closely it is necessary for the alleles controlling the polymorphism to exhibit dominance. Differences in received energy could make pale morphs disadvantageous compared with dark ones at low temperatures but advantageous at high ones. In spatially varied temperature conditions a polymorphism could be generated without predation. Density-dependent selection is again required.