Consequences of Intraspecific Competition and Environmental Variation for Selection in the Mustard Sinapsis arvensis: Contrasting Ecological and Evolutionary Perspectives.

Alternative models of plant life-history evolution differ in their views of how abiotic stress and competition interact to shape the evolution of plant life-history traits. To address this debate, which crosses traditional boundaries between community ecology and population biology, we grew wild turnip families from three selection histories in a field experiment in which we manipulated conspecific density and sun exposure. Hot spring conditions caused neutral shading to reduce drought stress, resulting in a greater mean and variance for lifetime fertility at low density and greater intensity of competition at high density. The variance in relative fitness among individuals or families was least in partial shade at low density. Prior selection under shade stress in the greenhouse reduced lifetime fitness in the less stressful partial-shade treatment under field conditions. Patterns of selection and predicted trait evolution were more similar between high and low densities than between the two light environments. Partial shade favored the proliferation of large leaves early in development, especially at high density. Selection in the stressful full-sun treatment favored reduced pathogen susceptibility at both densities and early flowering at low density. Because direct selection on traits changed principally in magnitude rather than in direction, genetic correlations for fitness were generally positive between light and density treatments. Greater intraspecific competition led to more rapid predicted trait evolution in the partial-shade environment but not in the stressful full-sun treatment.