Abstract
Over the next century, it is expected that the concentration of CO 2 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 CO 2 enrichment through genetic change. Here we describe microalgae isolated from the soil of natural CO 2 springs and compare these strains with lines of Chlamydomonas that were selected at elevated CO 2 in the laboratory. Both the laboratory and natural populations failed to evolve specific adaptations to elevated CO 2 , and contain populations that grow poorly at ambient levels of CO 2 . Laboratory and CO 2 spring populations also include lines whose growth rates are insensitive to CO 2 . This demonstrates that, although laboratory selection experiments use simplified environments, the evolutionary responses that are seen following long‐term CO 2 enrichment correspond to those found in natural populations that have experienced similar conditions.