Rapid evolution of unimodal but not of linear thermal performance curves in Daphnia magna

Species may cope with warming through both rapid evolutionary and plastic responses. While thermal performance curves (TPCs), reflecting thermal plasticity, are considered powerful tools to understand the impact of warming on ectotherms, their rapid evolution has been rarely studied for multiple traits. We capitalized on a 2-year experimental evolution trial in outdoor mesocosms that were kept at ambient temperatures or heated 4 degrees C above ambient, by testing in a follow-up common-garden experiment, for rapid evolution of the TPCs for multiple key traits of the water flea Daphnia magna. The heat-selected Daphnia showed evolutionary shifts of the unimodal TPCs for survival, fecundity at first clutch and intrinsic population growth rate toward higher optimum temperatures, and a less pronounced downward curvature indicating a better ability to keep fitness high across a range of high temperatures. We detected no evolution of the linear TPCs for somatic growth, mass and development rate, and for the traits related to energy gain (ingestion rate) and costs (metabolic rate). As a result, also the relative thermal slope of energy gain versus energy costs did not vary. These results suggest the overall (rather than per capita) top-down impact of D. magna may increase under rapid thermal evolution.

Automated summary

Species can adapt to warming through rapid evolution and plastic responses. In this study, we investigated the rapid evolution of thermal performance curves (TPCs) for multiple traits in the water flea Daphnia magna through a 2-year experimental evolution trial. The results showed that heat-selected Daphnia exhibited evolutionary shifts in TPCs for survival, fecundity, and population growth rate towards higher optimum temperatures, indicating a better ability to maintain fitness at high temperatures. However, there was no evolution observed in TPCs for somatic growth, mass, and development rate, as well as energy gain and cost-related traits.