Evidence of Adaptive Divergence in Plasticity: Density- And Site-Dependent Selection on Shade-Avoidance Responses in Impatiens Capensis

Abstract

We investigated the conditions under which plastic responses to density are adaptive in natural populations of Impatiens capensis and determined whether plasticity has evolved differently in different selective environments. Previous studies showed that a population that evolved in a sunny site exhibited greater plasticity in response to density than did a population that evolved in a woodland site. Using replicate inbred lines in a reciprocal transplant that included a density manipulation, we asked whether such population differentiation was consistent with the hypothesis of adaptive divergence. We hypothesized that plasticity would be more strongly favored in the sunny site than in the woodland site; consequently, we predicted that selection would be more strongly density dependent in the sunny site, favoring the phenotype that was expressed at each density. Selection on internode length and flowering date was consistent with the hypothesis of adaptive divergence in plasticity. Few costs or benefits of plasticity were detected independently from the expressed phenotype, so plasticity was selected primarily through selection on the phenotype. Correlations between phenotypes and their plasticity varied with the environment and would cause indirect selection on plasticity to be environment dependent. We showed that an appropriate plastic response even to a rare environment can greatly increase genotypic fitness when that environment is favorable. Selection on the measured characters contributed to local adaptation and fully accounted for fitness differences between populations in all treatments except the woodland site at natural density.

Natural selection on light response curve parameters in the herbaceous annual, Impatiens capensis

“…Differences in shade-avoidance morphology have been shown to be adaptive in the source populations from which the RILs used in this experiment were derived (e.g., Donohue et al 2000). However the fitness differences described by Donohue et al (2000) between open and woodland genotypes planted in the woodland site could not be entirely explained by selection on shade avoidance traits or flowering time, suggesting that local adaptation might also have favored shade-tolerant physiological traits in the woodland site.…”

“…However the fitness differences described by Donohue et al (2000) between open and woodland genotypes planted in the woodland site could not be entirely explained by selection on shade avoidance traits or flowering time, suggesting that local adaptation might also have favored shade-tolerant physiological traits in the woodland site. As a result, we expected that natural selection would also favor different combinations of light curve traits in the sun-shade treatments, which were Abbreviations are as in Table 3 Source designed to mimic light environments of the populations from which they were derived.…”

“…(Balsaminaceae), is an annual, selfcompatible herb of North American deciduous forests and wetlands and is an excellent model for studying the adaptive nature of light responses. Populations occur naturally under a wide range of light conditions, and earlier studies have documented substantial within and among population variation in many morphological, life-history, and physiological straits (Schemske 1984;Waller 1984;Schoen et al 1986;Schoen and Latta 1989;Argyres and Schmitt 1991;Schmitt 1993;Bennington and McGraw 1995;Dudley and Schmitt 1996;Paoletti and Holsinger 1999;Donohue et al 2000Donohue et al , 2001Heschel and Hausmann 2001;Heschel et al 2002). In natural populations, seedlings typically emerge early in spring under high light conditions, and grow rapidly prior to canopy closure.…”

“…Because I. capensis does not close its stomata at mid-day (Schulz et al 1993), gas exchange measurements can be made from 10 a.m. until 4 p.m. Most importantly, the populations from which the inbred lines used here are derived have already been shown to be locally adapted to different light conditions (Schmitt 1993;Dudley and Schmitt 1996;Donohue et al 2000).…”

“…During the last week of June plants were harvested: final height, reproductive fitness, and aboveground biomass were measured for all individuals. For each plant, reproductive fitness was estimated as the total number of flowers, fruits, and pedicels (Donohue et al 2000). The duration of the experiment approximated a short growing season for I. capensis; short seasons can occur in natural populations because of unpredictable summer droughts (M.S.…”