Over the next century, it is expected that the concentration of CO2 in the atmosphere will roughly double (Watson et al., 2001, Climate Change 2001: the Scientific Basis, Intergovernmental Panel on Climate Change, Geneva). Microbial populations, which have large population sizes and short generation times, may respond to CO2 enrichment through genetic change. Here we describe microalgae isolated from the soil of natural CO2 springs and compare these strains with lines of Chlamydomonas that were selected at elevated CO2 in the laboratory. Both the laboratory and natural populations failed to evolve specific adaptations to elevated CO2, and contain populations that grow poorly at ambient levels of CO2. Laboratory and CO2 spring populations also include lines whose growth rates are insensitive to CO2. This demonstrates that, although laboratory selection experiments use simplified environments, the evolutionary responses that are seen following long-term CO2 enrichment correspond to those found in natural populations that have experienced similar conditions.