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Many populations have evolved in response to laboratory environments (lack of predators, continual food availability, etc.). Another potential agent of selection in the lab is exposure to constant thermal environments. Here, we examined changes in growth, critical thermal maximum (CT_max), and food consumption under constant (25 °C) and fluctuating (22–28 °C and 19–31 °C) conditions in two populations of fathead minnows, Pimephales promelas: one that has been kept in a laboratory setting for over 120 generations (~40 years) and a corresponding wild one. We found that under thermal fluctuations, domesticated fathead minnows grew faster than their wild counterparts, but also exhibited lower thermal tolerance. Food consumption was significantly higher in the lab population under the constant and large fluctuation thermal treatments. Our results suggest that the lab population has adjusted to the stable …