Adaptation through natural selection may be the only means by which small and fragmented plant populations will persist through present day environmental change. A population’s additive genetic variance for fitness (V_A(W)) represents its immediate capacity to adapt to the environment in which it exists. We evaluated this property for a population of the annual legume Chamaecrista fasciculata through a quantitative genetic experiment in the tallgrass prairie region of the Midwest USA, where changing climate is predicted to include more variability in rainfall. To reduce incident rainfall, relative to controls receiving ambient rain, we deployed rain exclusion shelters. We found significant V_A(W) in both treatments. We also detected a significant genotype-by-treatment interaction for fitness which suggests that the genetic basis of the response to natural selection will differ depending on precipitation. For the trait specific leaf area, we detected maladaptive phenotypic plasticity and an interaction between genotype and environment. Selection for thicker leaves was detected with increased precipitation. These results indicate capacity of this population of C. fasciculata to adapt in situ to environmental change.

Fitness (number of seeds produced) and trait data (specific leaf area) were collected in an urban grassland in Minneapolis, Minnesota on a pedigreed population of the prairie annual legume, Chamaecrista fasiculata (partridge pea). Fitness was anlyzed in R using aster models. Quantitative genetic variance components of specific leaf area were estimated using the program Quercus (https://cbs.umn.edu/academics/departments/eeb/quercus). See manuscript for details.