Environmental stress does not increase the mean strength of selection

Abstract

A common intuition among evolutionary biologists and ecologists is that environmental stress will increase the strength of selection against deleterious alleles and among alternate genotypes. However, the strength of selection is determined by the relative fitness differences among genotypes, and there is no theoretical reason why these differences should be exaggerated as mean fitness decreases. We update a recent review of the empirical results pertaining to environmental stress and the strength of selection and find that there is no overall trend towards increased selection under stress, in agreement with other recent analyses of existing data. The majority of past studies measure the strength of selection by quantifying the decrease in fitness imposed by single or multiple mutations in different environments. However, selection rarely acts on one locus independently, and the strength of selection will be determined by variation across the whole genome. We used 20 inbred lines of Drosophila melanogaster to make repeated fitness measurements of the same genotypes in four different environments. This framework allowed us to determine the variation in fitness attributable to genotype across stressful environments and to calculate the opportunity for selection among these genotypes in each stress. Although we found significant decreases in mean fitness in our stressful environments, we did not find any significant differences in the strength of selection among any of the four measured environments. Therefore, in agreement with our updated review, we find no evidence for the oft-cited verbal model that stress increases the strength of selection.

Illustrating the importance of meta‐analysing variances alongside means in ecology and evolution

“…Developmental stress effects on phenotype and fitness have been studied often. For example, studies have investigated the effects of different developmental stressors on morphology and coloration (Tschirren et al ., 2009), attractiveness (Kahn et al ., 2012), social network position (Boogert et al ., 2014), telomere dynamics (Grunst et al ., 2019) and fitness (Arbuthnott & Whitlock, 2018). Several reviews and meta‐analyses have attempted to synthesize how different developmental stressors influence phenotype and fitness.…”

Dietary stress increases the total opportunity for sexual selection and modifies selection on condition‐dependent traits

“…Numerous studies have reported that individuals whose condition is experimentally manipulated exhibit reduced mean trait expression and reproductive success, leading to an overall increase in the opportunity for sexual selection (Zikovitz & Agrawal ; Martinossi‐Allibert et al ). However, when the whole population experiences a reduction in the resources available for reproduction, whether the opportunity for selection increases will largely depend on the effect of resources on the variance in the expression of sexually selected traits, rather than on their mean (Arbuthnott & Whitlock ; Fox et al ). With a few notable exceptions (David et al ; Howie et al ), studies that test for condition dependence in sexually selected traits tend to focus on trait means rather than variances (Jennions et al ; Cotton et al ).…”

Temperature as a modulator of sexual selection

“…Our focus in this meta‐analysis is to explore temperature effects per se , beyond any effects that temperature may have due to subjecting individuals/populations to a new environment to which they are maladapted. Environmental stress can modulate the strength of sexual selection in a variety of ways (Arbuthnott & Whitlock, 2018), for example increasing sexual selection if the variability in fitness is inflated when populations are pushed off their fitness peak (Martinossi‐Allibert et al ., 2017, 2018). As such, drastic temperature changes can modulate sexual selection not due to specific effects of temperature but by imposing a stressful environment, much in the same way as a pollutant or a sharp change in another abiotic factor would.…”