Dwarfing during periods of mass extinction is balanced by increasing body sizes during periods with background extinctions only, a trend known as Cope's rule. The existence of the two trends indicates that body size evolution may be controlled by deterministic natural selection. While deterministic selection goes beyond the 20th century consensus that natural selection is a purely a posteriori historical process, it is illustrated that large-scale evolution of life-history traits like body mass might indeed be driven by deterministic selection that arises a priori from the self-replication process of living organisms. It is discussed how deterministic selection by density dependent competitive interactions and the energetic state of organisms may explain not only dwarfing during periods of mass extinction and body size increase during stable environmental conditions, but also more general life-history evolution including traits like sexual reproduction and eusocial colonies. The predicted direction of body size change is determined by the resource availability to the species, indicating that the proposed framework may enhance our understanding of organism responses to ecological changes.