Effects of Short‐Term Selection Along Environmental Gradients on Variation in Populations of Amaranthus Retroflexus and Abutilon Theophrasti

This study examines the effects of short—term selection along resource gradients on variation within plant populations. Populations of Amaranthus retroflexus and Abutilon theophrasti were subjected to four generations of selection in the following treatments: soil moisture (low, moderate, or high), diversity of competitor species (low, moderate, or high), and density (low or high). In several cases such selection resulted in significant divergence of the population from its original configuration. The changes occurred in such characters as photosynthetic rate, reproductive allocation, flowering time, root/shoot allocation, and shoot biomass. In all cases where significant changes in overall level of variation occurred, these were in the direction of reduced variation. The rapidity and nature of these changes differed according to the level and type of treatment. Reductions in variation occurred almost uniformly in the density treatments. In contrast, changes in variation were most unpredictable in the diversity treatments, owing to a high level of variability in the competitive background that was apparently associated with generation—to—generation variability in light and temperature. In some generations Abutilon theophrasti was greatly suppressed by heterosopecific competitors. This resulted in a significant reduction in variation in spite of high levels of environmental variability. Thus environmental variation in itself may not be sufficient to maintain variation in the population of extreme fluctuation in population size results in genetic bottlenecks. Levels of morphological and physiological variation were maintained in all but a low— and a moderate—moisture line of Amaranthus retroflexus, but the diversity of electrophoretically marked genotypes declined in all moisture treatments. The loss of variation at moderate soil moistures may be due to an overlying gradient of conspecific competition, a situation which cannot be avoided in even the simplest of gradients. Despite significant changes in several characters and in levels of overall variation, technological and analytical tools were found to be inadequate to determine if these changes led to significant and consistent differences in the response patterns of the species along these gradients.