Variation in growth rate in a natural assemblage of unicellular green soil algae

Unicellular, motile, phototropic green algae were extracted from soil samples taken at metre intervals along a 25-m transect in a wheat field. The vegetative growth of 61 randomly selected isolates (henceforth called spores) was measured in dark and light conditions, and at high and low nutrient concentrations in liquid media. The among-spore variance was dominated by a spore-by-environment interaction. The increase of among-spore variance with distance was detectable but slight, showing that most of the diversity found on the transect was present at a scale of metres. A mixture of all spores, constituting the founder assemblage, was propagated for 50-70 generations in three environments to study the sorting of variation initially present in the assemblage. Adaptation to the new environments was measured by improvement in growth relative to the founder assemblage. All three sets of lines became adapted to their selection environment, although the extent of the advance depended on the amount of among-spore variance in the founder assemblage. Selection for increased growth in the dark, or at high nutrient conditions in the light, caused the assemblage to evolve to the limit of the variation initially present, strongly reducing among-spore variance as a consequence. At low nutrient concentrations selection was less effective, and there was little reduction of the among-spore variance. The correlated responses of each group, again relative to the founder assemblage, were measured over the entire experimental range. Correlated responses were negative when performance in widely different (light and dark) environments was compared, but positive for more similar environments (high and low nutrient concentrations).