Examining genomic and plant level responses to rising CO₂ allows for incorporation of the nature of the responses and the mechanisms controlling these responses when scaling their impact to the community and ecosystem levels. Also necessary for accurately scaling the effects of elevated CO₂ on plants to higher levels of organization over long time periods is a comprehensive understanding of changes in plant evolution with elevated CO₂. Therefore, the responses of traits that impact plant fitness are important for projecting the long-term effects of elevated CO₂. One such trait is the transition from vegetative to reproductive growth, because the timing of this transition can impact the fitness and yield of plants in natural and agro-ecosystems. Furthermore, the onset of reproduction commonly marks the beginning of senescence, thus changes in reproductive timing may influence productivity at higher levels of organization through altered size at reproduction. Increasing CO₂ both accelerates and delays the timing of reproduction in many plant species. Previously we selected several genotypes of Arabidopsis for high fitness over five generations at elevated CO₂ to examine the physiological mechanisms that confer increased fitness at elevated CO₂. When grown at current and elevated CO₂, growth at elevated CO₂ significantly delayed the reproductive timing of the genotype that exhibited the largest increase in fitness with selection. However, a genotype that served as a control for the selection process did not exhibit changes in reproductive timing. Using gene expression studies we have made significant progress in determining the mechanisms through which elevated CO₂ delays flowering in the selected genotype. To date, we have confirmed our visible observations of delayed flowering with elevated CO₂ at the molecular level. In addition, we have compelling evidence to suggest that elevated CO₂ acts through the autonomous floral signaling pathway of Arabidopsis to delay reproduction.