Selective history is thought to constrain the extent and direction of future adaptation by limiting access to genotypes that are advantageous in a novel environment. Populations of Chlamydomonas previously selected at high CO₂ were either backselected at ambient levels of CO₂, or selected at levels of CO₂ that last occurred during glaciation in the Pleistocene. There was no effect of selective history on adaptation to either level of CO₂, and the high CO₂ phenotypes were evolutionarily reversible such that fitness in ambient CO₂ returned to values seen in controls. CO₂ uptake affinity improved relative to the ancestor in both ambient and glacial CO₂, although wild-type regulation of CO₂ uptake, which deteriorated during previous selection at high CO₂, was not restored by selection at lower levels of CO₂. Trade-offs in both CO₂ uptake affinity and growth were seen after selection at any given level of CO₂. Adaptation to ambient and glacial-era levels of CO₂ produced a range of phenotypes, suggesting that chance rather than selective history contributes to the divergence of replicate populations in this system.