Phenotypic plasticity describes the ability of organisms to modulate phenotype in response to the environment and can provide necessary genetic variation to fuel rapid evolution. However, because plasticity can expand the range of conditions that permit high survival and fecundity of individuals, plasticity itself can also be considered to be the adaptive response (e.g., Baldwin’s effect, genetic assimilation). Because plasticity and genetic adaptation can generate outcomes that appear similar, the underlying processes of plasticity and adaptation can be masked within that population. Therefore, disentangling the role of phenotypic plasticity in adaptive evolution remains one of the most important challenges for understanding and predicting evolutionary outcomes in response to environmental change. Here, we present a study that investigates the role of plasticity in generating different frog phenotypes by comparing how life history endpoints differ between coastal (salt-exposed) and inland (salt-naïve) frog populations after hatching and developing in different salinity environments. Additionally, we relate our experimental findings to conductivity data from wetlands that host coastal frog populations collected over the past four years using data loggers.
Results/Conclusions
We find that for both coastal and inland populations of Hyla cinerea, the natal environment affects ability to tolerate moderate levels of saltwater exposure. Exposing eggs to salt stress triggers differences in morphology, survival, and development, but typically in a negative direction. Indeed, coastal and inland individuals that hatched from eggs laid in freshwater tended to grow faster, metamorphose at larger sizes, and survive better than individuals from eggs laid in saltwater. However, across nearly every measured trait (e.g., growth rate, size at metamorphosis, survival to metamorphosis), we find that coastal tadpoles consistently perform better than inland tadpoles in higher salinities. In general, coastal tadpoles hatched in the saline natal environment do better than the inland eggs hatched in freshwater. Our results suggest that there are both gene and environment interactions underlying salt-tolerance in coastal frogs, and these interactions are consistent with Baldwin’s effect. Baldwin’s effect (which describes adaptive evolution by means of evolving reaction norms) is hypothesized to be prevalent in highly variable habitats. We show that salinity in the habitats from which coastal frogs originate are highly variable across time and space.