Is phenotypic plasticity use-it-or-lose-it? Exploring genetic assimilation of salinity-plastic traits across threespine stickleback (<em>Gasterosteus aculeatus</em>) populations

Understanding the response of phenotypically plastic traits to novel environments is critical to predicting evolutionary dynamics. We lab-reared multiple wild-caught threespine stickleback (Gasterosteus aculeatus) populations under a range of salinities and recorded the plasticity of morphological, physiological, and fitness-related traits. Following freshwater colonisation, populations showed a short-term increase or maintenance of salinity tolerance breadth (i.e., consistency of survival across salinities) – while a subsequent loss of salinity tolerance occurred in most, but not all, populations over the longer term. Despite variability amongst physiological and morphological responses to increased salinity across populations, we found that resolution of generalist-specialist trade-offs may drive plasticity loss: less plastic populations grew faster in freshwater. Our findings establish that predicted Simpson-Baldwin dynamics (plasticity gain and then subsequent loss under relaxed selection) can apply to plastic traits in natural populations, although the underlying mechanisms may be variable, even within species.