Effects of plasticity and genetic divergence in phenotypic trait expression of sulfide spring fishes

Natural environments vary, and organisms cope with this variation in two general ways: local adaptation and phenotypic plasticity, although these strategies often overlap and interact. This study explored how local adaptation, phenotypic plasticity, and their interactions shaped phenotypic variation in populations of Poecilia mexicana, an extremophile fish living in adjacent but ecologically divergent habitats. By comparing populations from the wild with fish raised in a common-garden environment, we evaluated how genetic differentiation between populations and plasticity contributed to the phenotypic variation observed in nature. We quantified variation in the size of six organs (brain, eyes, gills, heart, liver, and gastrointestinal tract), routine metabolic rate, and body shape. We found evidence for genetic differences between populations impacting the expression of the majority of traits, in addition to or in interaction with phenotypic plasticity and other predictor variables. Overall, our results suggest that trait divergence between populations was at least in part driven by evolutionary change and not just merely by plasticity induced by environmental differences between habitats. Future studies will have to rigorously test whether evolutionary divergence was caused by natural selection and what traits represent adaptations to the different ecological conditions.