A century of isolation in contrasting environments drives evolutionary divergence in a small population of salmonids.
Résumé
Small and isolated populations experience unique evolutionary dynamics shaped by selection, phenotypic plasticity, and potentially strong genetic drift. Understanding how multiple traits evolve in such populations is important for predicting their adaptive potential in response to climate change. Here, we investigate the divergence of metabolic, morphological and growth-related traits in two allopatric populations of Alpine charr (Salvelinus umbla) isolated for a century in ecologically contrasting lakes. We combined a common garden experiment with a quantitative genetic approach to disentangle the relative contributions of genetic divergence and plasticity to phenotypic differentiation. Population genetic differentiation was estimated using microsatellite markers and we applied Qst-Fst comparisons to test whether phenotypic divergence resulted from adaptive (e.g., selection) or neutral (e.g., drift) processes. Our results revealed low neutral genetic differentiation (Fst = 0.017), suggesting limited drift, and Qst-Fst comparisons indicated signatures of divergent selection for several traits. Adults showed an important divergence in body shape and growth trajectories, potentially underlying a high environmental plasticity due to resource availability and temperature variation. In juveniles reared under common garden conditions, we found a substantial differentiation in metabolic rate, its allometric scaling and morphological traits likely related to trophic or visual functions (as indicated by an elevated Qst), suggesting early-life divergent selection. These differences may reflect adaptation to distinct thermal regimes for metabolic traits, and to contrasting resource availability or foraging demands for morphological traits. This study provides empirical evidence of rapid trait evolutionary divergence in isolated populations, highlighting the role of both selection in evolutionary trajectories and phenotypic plasticity to phenotypic differentiation across environments. These findings highlight the importance of small, isolated populations in understanding adaptive potential in a changing climate.