The paper by Kingsolver and Huey explores three well-known empirical rules in the context of body size, temperature, and fitness in ectotherms: "Bigger is better," "Hotter is smaller," and "Hotter is better." The authors summarize evidence supporting these rules, discuss counterexamples, and propose a conceptual framework to integrate their effects. 1. **Bigger is better**: Larger body size is generally associated with higher fitness, including survival, fecundity, and mating success. This rule is supported by strong directional selection on size in natural populations, particularly in terrestrial insects, reptiles, and annual plants. 2. **Hotter is smaller**: Development at higher temperatures leads to smaller adult body sizes. This phenotypic plasticity is observed in multiple taxa, with over 83% of studies supporting this rule. However, the thermal reaction norms can evolve in response to selection, leading to exceptions. 3. **Hotter is better**: Genotypes or species with higher optimal temperatures tend to have higher maximal performance or fitness. This rule is based on thermodynamic arguments that reaction rates increase with temperature. Empirical evidence supports this rule, particularly when using the intrinsic rate of population increase ($r$) as a fitness metric. The authors highlight the importance of distinguishing between direct and indirect effects of size and temperature on fitness components, such as survival, fecundity, and mating success. They also emphasize the need to consider different fitness metrics (e.g., $r$ vs. $R_0$) in various ecological contexts. The rules interact dynamically, sometimes synergistically and sometimes antagonistically, influencing evolutionary outcomes. The paper concludes by discussing the limitations and biases in the data, the need for more comprehensive studies, and the importance of understanding how fitness components combine to determine overall fitness. It also suggests that these rules can be used to predict patterns of selection in response to climate changes or geographic variations.The paper by Kingsolver and Huey explores three well-known empirical rules in the context of body size, temperature, and fitness in ectotherms: "Bigger is better," "Hotter is smaller," and "Hotter is better." The authors summarize evidence supporting these rules, discuss counterexamples, and propose a conceptual framework to integrate their effects. 1. **Bigger is better**: Larger body size is generally associated with higher fitness, including survival, fecundity, and mating success. This rule is supported by strong directional selection on size in natural populations, particularly in terrestrial insects, reptiles, and annual plants. 2. **Hotter is smaller**: Development at higher temperatures leads to smaller adult body sizes. This phenotypic plasticity is observed in multiple taxa, with over 83% of studies supporting this rule. However, the thermal reaction norms can evolve in response to selection, leading to exceptions. 3. **Hotter is better**: Genotypes or species with higher optimal temperatures tend to have higher maximal performance or fitness. This rule is based on thermodynamic arguments that reaction rates increase with temperature. Empirical evidence supports this rule, particularly when using the intrinsic rate of population increase ($r$) as a fitness metric. The authors highlight the importance of distinguishing between direct and indirect effects of size and temperature on fitness components, such as survival, fecundity, and mating success. They also emphasize the need to consider different fitness metrics (e.g., $r$ vs. $R_0$) in various ecological contexts. The rules interact dynamically, sometimes synergistically and sometimes antagonistically, influencing evolutionary outcomes. The paper concludes by discussing the limitations and biases in the data, the need for more comprehensive studies, and the importance of understanding how fitness components combine to determine overall fitness. It also suggests that these rules can be used to predict patterns of selection in response to climate changes or geographic variations.