Inclusive fitness forces of selection in an age-structured population

Abstract Current evolutionary theories of senescence predict that the force of selection on survival will decline from maturity to zero at the age of last reproduction, and the force of selection on reproduction will decline monotonically from birth. These predictions rest upon the assumption that individuals within a population do not interact with one another. This assumption, however, is violated in social species, where an individual’s survival and/or reproduction may shape the fitness of other group members. In such species, it is inclusive fitness that natural selection optimises. Yet, it remains unclear how the forces of selection on survival and reproduction might be modified when inclusive fitness, rather than population growth rate, is considered the appropriate metric for fitness. Here, we derive inclusive fitness forces of selection for hypothetical populations of social species. We show that selection on survival is not always constant before maturity, and can remain above zero in post-reproductive age classes, contrary to conventional models of senescence. We also show how the trajectory of the force of selection on reproduction does not always decline monotonically from birth, as predicted by classical theory, but instead depends on the balance of benefits to direct fitness and costs to indirect fitness. Our theoretical framework provides the unique opportunity to expand our understanding of senescence across social species, with important implications to species with variable life histories.