KINGSOLVER, J.G.*; MASSIE, K.R.; RAGLAND, G.J.; SMITH, M.H.; Univ. of North Carolina, Chapel Hill: Breaking the temperature-size rule: Rapid population divergence in thermal reaction norms

The temperature�size rule is a common pattern of phenotypic plasticity in which higher temperature during development results in a smaller adult body size (i.e., a thermal reaction norm with negative slope). We examine genetic and parental contributions to population differentiation in thermal reaction norms for size, development time and survival in the Cabbage White Butterfly Pieris rapae, for two geographic populations (Seattle WA and Chapel Hill NC) that have diverged within the past 100-150 years. We used split-sibship experiments with two temperature treatments (warm and cool) for P. rapae for each population. Mean adult mass was significantly greater in NC than in WA populations for both temperature treatments; mean size decreased with temperature (the temperature-size rule) for the WA population, but size increased with temperature for the NC population. There were significant parental contributions to thermal reaction norms for survival, resulting in decreased mean survival for the WA population in warm conditions. Our study shows that the temperature-size rule and related thermal reaction norms can evolve rapidly within species in natural field conditions. Rapid evolutionary divergence argues against the existence of a simple, general mechanistic constraint as the underlying cause of the temperature-size rule.