Effects of development and parental care on Hamilton’s force of selection

The force of selection describes the sensitivity of population growth to changes in life history parameters, with a focus usually on the survival probabilities from one age class to the next. Importantly, according to Hamilton the force of selection generally decreases after the onset of reproduction, thereby providing a possible explanation for patterns of senescence. A second characteristic feature is that the force of selection remains constant up to the age of first reproduction. This latter observation, however, rests on the assumption that offspring become independent from their parents right after birth. I show here in a minimal model that if offspring are fully reliant on their parents, either during early embryonal development or via parental care at later stages, and during this time prevent their parents from entering a new bout of reproduction, the force of selection on offspring survival generally increases up until the age at which offspring become independent. This provides a possible explanation for the commonly observed pattern of decreasing mortality during early ontogeny. Furthermore, genes acting during recurrent life stages are observed to experience a heightened force of selection compared with genes that act strictly age specifically, demonstrating the need to develop a mechanistic understanding of gene activation patterns through which to consider life history evolution.