Streambed microstructure predicts evolution of development and life history mode in the plethodontid salamander Eurycea tynerensis

We find streambed microstructure (sediment particle size, type and degree of sorting) to be highly correlated with life-history mode. Eurycea tynerensis is paedomorphic in streams containing large chert gravel, but metamorphoses in nearby streams containing poorly sorted, clastic material such as sandstone or siltstone.Deposits of large chert gravel create loosely associated streambeds, which provide access to subsurface water during dry summer months. Conversely, streambeds composed of more densely packed sandstone and siltstone sediments leave no subterranean refuge when surface water dries, presumably necessitating metamorphosis and use of terrestrial habitats. This represents a clear example of the relationship between microhabitat structure and evolution of a major developmental and life history trait, and has broad implications for the role of localized ecological conditions on larger-scale evolutionary processes.The ability to express alternate phenotypes enables populations to colonize and persist in variable environments [1-4]. However, the role of phenotypic plasticity in the evolution of traits, and subsequent diversification and speciation, is understood in relatively few instances [4]. Populations experiencing a given environmental regime across generations may undergo directional selection, from a highly plastic trait to a single condition [4-6], thus initiating phenotypic divergence among populations that occupy different environments. Furthermore, selection acting on reproductive traits may lead to reproductive isolation among populations. Especially for organisms with complex life cycles, local selection involving major developmental shifts can produce rapid changes in ecology, morphology and life history among populations, and may hasten divergence and speciation [7-9].Amphibians exhibit diverse life history strategies that enable them to exploit both aquatic and terrestrial ecosystems successfully [10-13]. Most amphibians have a free-living aquati