Replicated phenotype-environment matching among multiple populations of the same species can indicate an adaptive response to a shared environmental gradient. Populations could also exhibit unique responses to a shared environmental gradient that accounts for divergence or site-specific conditions. We test for phenotype-environment matching in physid snail-shell shape between stream and pond habitats in three adjacent watersheds. Snails in streams experienced high flow conditions compared those in ponds. Based on these abiotic conditions, we predicted that snails from streams would exhibit elongate and narrow shells, while those from ponds would have rotund shells.
Results/Conclusions
We found strong evidence of phenotype-environment matching between stream and pond habitats among populations of snails. There was also evidence of unique responses of populations to the environmental gradient. The observed shell-shape variation supported our predictions. Snails from stream environments, regardless of population, exhibited elongate and narrow shells compared to snails collected from pond habitats. This pattern of shell-shape variation in physid snails is similar to the one observed between entry-based and crushing predators. Similar solutions to different environmental gradients may represent constraints on the ability of organisms to produce optimal phenotypes.