Experimental evolution in Chlamydomonas .2. Genetic variation in strongly contrasted environments

Experimental populations of Chlamydomonas were selected in Light (photoautotrophic) or Dark (heterotrophic) environments. Each population was a clone, founded by a single spore and propagated vegetatively thereafter. A heterogeneous environment was simulated by mixing Light and Dark lines in each growth cycle and redistributing them between the two environments in the next cycle. Some lines maintained permanently in the Dark evolved greatly increased growth within fewer than 300 generations, at the expense of reduced growth in the Light. Lines maintained in both Light and Dark environments evolved a negative genetic correlation between Light and Dark growth, and displayed more genetic variance of fitness than lines maintained in either environment exclusively. It is possible that genetic variance near mutation-selection balance is greater in heterogeneous environments because selection is weaker. However, the evolution of distinctly specialized lineages in these experiments suggests that in the conditions of batch culture a cost of adaptation creates negative frequency-dependent selection that maintains genetic variance. Genetic variance was greater in the more permissive environment (Light) than in the more restrictive environment (Dark).