Rates of gene flow in a freshwater snail and the evolution of phenotypic plasticity

The evolution of phenotypic plasticity requires a number of conditions. Selection of plasticity is favoured when the organism experience environmental change, costs are low and cues are reliable about the environmental heterogeneity. However, organisms living in stable environments, not showing constitutive traits but a large amount of plasticity, are predicted to demonstrate high rates of gene flow in order for selection to favour the evolution of phenotypic plasticity, which accordingly should provide weak genetic structures across populations. We used the pulmonate freshwater gastropod Radix balthica, a species with known and considerable shell shape variation due to predator-induced plasticity, and used amplified fragment length polymorphism markers to test if the rate of gene flow can explain the evolution of phenotypic plasticity. Since R. balthica inhabit water bodies with different but consistent predator regimes, we envisaged a large dispersal rate. However, we found a contradictory result with clear population structures, even among adjacent ponds in southern Sweden. We discuss this apparent paradox in contrast to the evolution of ecotype formation, colonization mechanisms that have the potential to reduce gene flow and, in the context of costs of plasticity, we consider new perspectives about relaxed and variable selection that may drive the evolution of phenotypic plasticity.