Natural selection on and predicted responses of ecophysiological traits of swamp milkweed (Asclepias incarnata)

1. Given the rapid and reversible impact of the environment on plant ecophysiological traits, it has been speculated that such traits may have low heritabilities, high phenotypic plasticity and experience selection which is highly variable. 2. We estimated heritabilities, phenotypic plasticity and natural selection on five ecophysiological traits (carbon : nitrogen (C : N) ratio, leaf water content, specific leaf area (SLA), trichome density and water use efficiency (WUE)) of swamp milkweed Asclepias incarnata. In a 2-year field experiment, we studied 49 full-sibling families in a common garden and assessed phenotypic and genotypic selection. 3. Consistent evidence indicated selection for increased leaf trichome density and reduced WUE. Less strong evidence suggested selection for reduced SLA and leaf water content, and there was no selection on C : N of leaves. 4. Although the ecophysiological traits showed many phenotypic and genetic correlations, we found little evidence that these correlations would constrain adaptive evolution. 5. To evaluate the phenotypic plasticity of the same traits, we conducted a separate growth chamber experiment and found evidence that leaf water content, SLA and C : N responded to environmental conditions (herbivory, water stress, nutrient addition). 6. Synthesis. We have demonstrated natural selection on ecophysiological traits and their probable response to selection with a genotypic selection analysis. We cautiously reject the notion that these traits lack heritable variation and that genetic correlations among them constrain their response to selection. Thus, plants such as A. incarnata appear capable of adaptive responses in their ecophysiological traits.