Adaptation and plasticity in life-history theory: How to derive predictions

Why is life paced so differently across as well as within organisms? Can one expect across-species patterns to be repeated within a species too, among individuals? The answer to these questions requires understanding conditions under which reaction norms evolve. We provide an overview of what we believe to be understudied areas of life-history theory, to foster theoretical work and to help deriving predictions for the evolution of human reaction norms. We discuss both why one might expect reaction norms to be aligned with patterns across species, and why that expectation might sometimes fail. It is not impossible for environmental cues to shape life histories in the current generation, but compared with cue-independent genetic adaptation, the adaptive task is now more complex; cues may be unreliable or change in value with time; and parental strategies may differ between situations where offspring have the possibility to disperse to new habitats and situations where environmental conditions remain the same across generations. In that regard, we comment on the value of source-sink theory and on the importance of being specific about the way density regulation affects individual vital rates. We also remind the reader that adaptation does not necessarily optimize population growth rates when conflict between entities (e.g. between the two sexes) is a feature of the adaptive process. All these factors likely play an important role on the evolution of reaction norms, and we argue in favour of their more systematic inclusion in human life-history research.