Populations at risk of extinction due to climate change may be rescued by adaptive evolution or plasticity. Selective agents, such as introduced predators, may enhance or constrain plastic or adaptive responses to temperature. We tested responses of Daphnia to temperature by collecting populations from lakes across an elevational gradient in the presence and absence of fish predators (long-term selection). We subsequently grew these populations at two elevations in field mesocosms over two years (short-term selection), followed by a common-garden experiment at two temperatures in the lab to measure life-history traits. Both long-term and short-term selection affected traits, suggesting that genetic variation of plasticity within populations enabled individuals to rapidly evolve plasticity in response to high temperature. We found that short-term selection by high temperature increased plasticity for growth rate in all populations. Fecundity was higher in populations from fishless lakes and body size showed greater plasticity in populations from warm lakes (long-term selection). Neither body size nor fecundity were affected by short-term thermal selection. These results demonstrate that plasticity is an important component of the life-history response of Daphnia, and that genetic variation within populations enabled rapid evolution of plasticity in response to selection by temperature.