Evolutionary consequences of simulated global change: genetic adaptation or adaptive phenotypic plasticity

During the next century, natural and agricultural systems might need to adjust to a rapid increase in atmospheric CO₂ concentration and global temperature. Evolution of genotypes adapted to this global change could play a central role in plants' response. The main purpose of this study was to determine the relative importance of phenotypic and genotypic responses of plants to global change. To do so, we selected two populations of the short-lived Brassica juncea, one under ambient conditions and another one under conditions simulating global change. After seven generations of selection, differences between the two populations were examined using a reciprocal transplant garden. We monitored 14 different traits and found evidence for genetic adaptation only once, for vegetative biomass early in the growth cycle. Of the 14 traits, 11 responded plastically to the environment, but only one of these plastic changes had a possible adaptive value. Overall, the long-term evolutionary consequences of global change will depend on the response of fitness-related traits. None of the five reproductive traits measured showed any evolutionary responses. The main conclusion of our study is that Brassica juncea was apparently unable to respond evolutionarily to simulated global change either by genetic adaptation or by adaptive phenotypic plasticity. The limit to selection was apparently due to inbreeding depression induced by the harsh conditions of the predicted environment.