Quantitative genetics became central to population development, blending genetic parameters with demographic processes to explain growth, variability, and survival in populations. Researchers stressed estimating variance components, including genotypic and environmental variances, and incorporating genotype–environment interactions, while acknowledging substantial individual heterogeneity as a driver of population trajectories. Methodologically, there was a shift toward model-based approaches, including flexible growth functions and early computer-assisted simulations to test ideas about selection and population dynamics.