Why is body size more variable in stressful conditions: an analysis of a potential proximate mechanism

Abstract

Patterns of variability in quantitative traits across environmental gradients have received relatively little attention in evolutionary ecology. A recent meta-analysis showed that relative phenotypic variability in body size tends to decrease with improving environmental conditions. This pattern was explained by introducing the concept of upper threshold size to a general optimality model of individual growth but alternative explanations certainly exist. In particular, it is frequently observed in insects that variability in individual growth rates decreases with improving environmental conditions. Here we explore the effect of this phenomenon on environment-specific variability in adult sizes. A quantitative model shows that relative variability in adult sizes is independent of environmental quality if absolute variability in growth rates remains constant across the gradient of environmental quality. Deviations from this borderline case are definitely realistic in both directions. Both negative and positive relationships between relative variability of body size and environmental quality can thus be predicted to arise as a consequence of environment-specific variability in growth rates. The variability itself can be both genetic or environmental in its nature. We present empirical data which support both the assumptions and conclusions of our model-based analysis, as well as emphasize the advantages of controlled experiments for understanding the proximate sources of phenotypic variance.