From traits to trajectories: development, agency and a phenotypic gap in demography

Demography links ecology and evolution by connecting population dynamics with variation in fitness and selection. Yet, most demographic models represent phenotypes as fixed traits and environments as static backdrops, overlooking the developmental processes through which phenotypes are constructed. This creates a phenotypic gap in demographic theory: a disconnect between how phenotypes arise through development and how they are represented in demographic models. A development-centric perspective can address this gap by incorporating agency—the capacity of organisms to regulate development, reallocate energy, and modify their environments through feedback—into demographic thinking. State-of-the-art demographic models, including Integral Projection Models (IPMs) and Dynamic Energy Budget (DEB) IPMs, capture phenotypic outcomes and aspects of physiological development, but they do not explicitly represent how agential, feedback-driven developmental processes shape phenotypes and selection. I argue that progress can be made by integrating DEB-based, individual-based models with agent-based and decision-theoretic approaches, such as Markov decision processes, which represent organisms as active developmental systems embedded in feedback-rich environments. Coupling these approaches with evolutionary tools from quantitative and population genetics would yield a unified, development-centric demography. Using examples of context-sensitive developmental trajectories and intraspecific phenotypic diversification, I show how development-centric approaches reveal how energy allocation, developmental plasticity, and feedback shape phenotypic variation and demographic dynamics. A development-centric demography reframes population dynamics as a science of self-directed change and provides a stronger foundation for understanding eco-evolutionary feedback and forecasting population resilience under environmental change.