Evolution in stressful environments. I. Phenotypic variability, phenotypicselection, and response to selection in five distinct environmental stresses

Considerable debate has accompanied efforts to integrate the selective impa cts of environmental stresses into models of life-history evolution. This s tudy was designed to determine if different environmental stresses have con sistent phenotypic effects on Life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We cr eated lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes thigh boron, high salt, low lig ht, low water, or low nutrients) or under near-optimal conditions (control) . Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime pl ant fecundity and six phenotypic traits were measured for each plant. Throu ghout this greenhouse study, plants were grown individually and stresses we re applied from the early seedling stage through senescence. Although all s tresses consistently reduced lifetime fecundity and most size- and growth-r elated traits, different stresses had contrasting effects on flowering time . On average, stress delayed flowering compared to favorable conditions, al though plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not re spond phenotypically to poor environments by flowering earlier. Most stress es enhanced the evolutionary potential of the study population. Compared wi th near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantitie s were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenoty pic selection favored stress- avoidance traits: earlier flowering in ail fi ve stress regimes and faster seedling height growth in three stresses. Phen otypic correlations reinforced direct selection on these traits under stres s, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selecti on under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in st ressful environments leads to these alternative life-history strategies.