Carbon dioxide is an important selective agent in the evolution of plants. The rapid increase in the concentration of atmospheric carbon has given considerable impetus to studies exploring the impact of elevated carbon dioxide (eCO2) on plant populations. While eCO2 has been shown to affect plants at multiple levels of biological organization--from gene expression to community structure--our understanding of plant evolutionary dynamics in eCO2 environments is limited. A key step in elucidating evolutionary responses is characterizing the effects of novel genetic variation on patterns of phenotypic plasticity and integration among ecologically relevant traits. This approach can provide insights into the extent of variation on which selection may operate and reveal potential constraints on adaptive evolution. I designed a carbon dioxide supplementation system to study phenotypic responses to eCO2 in natural, mutagenized and recombinant inbred populations of the model flowering plant Arabidopsis thaliana. I found differentiation in both phenotypic integration and plasticity to eCO2 among natural populations. I also found that novel mutations, and to a limited extent recombination, increased genetic variation, altered patterns of covariance among traits, and significantly increased genetic variation in plasticity to eCO2. My results suggest that future atmospheric carbon concentrations may alter selection dynamics, and the accumulation of relatively few mutations may radically alter norms of reaction and genetic architecture in A. thaliana. I discuss the implications of these findings in light of recent insights from theoretical and empirical quantitative genetics, and identify approaches that may help advance our understanding of climate-driven evolution in plants.