The world is undergoing rapid environmental change, and organisms are growing up in environments very different to those in which they evolved. Predicting how organisms will cope with this unprecedented scale of change is a key priority of biological research. The proposed work will be a significant advance to the field by measuring the evolutionary potential of developmental thermal physiology across natural populations, testing an underlying mechanism mediating adaptation to a warmer climate: namely the thermal sensitivity of mitochondrial efficiency (ATP/O). Outcomes will advance both basic biological knowledge, and applied climate change responses which are of importance to EU citizens, by testing individual- and population- level variation to predict species-level responses to climate change, and identifying a potentially general mechanism for thermal adaptation that can focus conservation efforts. I will use a combination of field work with laboratory measurements and techniques on natural populations of a widely distributed freshwater fish, and meta-analytical techniques, to target the following novel objectives: Objective 1) Test for divergence in mitochondrial efficiency (ATP/O) and developmental thermal physiology across locally adapted populations spanning a wide latitudinal gradient; Objective 2) Identify how developmental temperature affects ATP/O; Objective 3) Determine how selection and heritability of developmental thermal physiology varies across populations, and how ATP/O may mediate temperature-dependent selection; Objective 4) Summarise the generality of patterns between ATP/O and environment across species using meta-analysis. This project will draw from and expand upon the combined research track-records and strengths of my work in early-life history and quantitative genetics, with that of hosts Prof. Metcalfe (subcellular physiology) and Prof. Seebacher (whole-animal physiology) to achieve my future research career goals.