• Chlamydomonas reinhardtii;
  • environmental grain;
  • generalist;
  • plasticity;
  • specialist;
  • versatility.

Specialists and two kinds of generalist were selected for in a genetically heterogeneous base population of the unicellular chlorophyte Chlamydomonas reinhardtii. The selection environments consisted of alternating periods of light and dark. When the environment remains constant (light or dark), specialists are expected to evolve; when the environment varies through time, generalists are expected to evolve. The kind of generalist that evolves depends on the period of environmental variation: versatile generalists capable of reversible responses to growth conditions are expected to evolve when the environment is fine-grained, whereas plastic generalists that respond irreversibly to the conditions of growth are expected to evolve when the environment is coarse-grained. The results indicate that specialists evolve in constant environments and generalists evolve in variable environments, as expected, but no evidence was found to support the idea that versatility and plasticity evolve in fine-grained and coarse-grained environments respectively. Moreover, the evolved generalists performed well in every environment and were insensitive to environmental variation. These results are interpreted to mean: (1) selection in the variable environments acted on the mean performance in each environment, rather than on the variance in performance across environments; (2) there was little cost to being a generalist.