Hormones and the physiological architecture of life history evolution

Hormones play key roles in the regulation of animal and plant life histories, particularly in the timing of transitions between prematurational stages and in the scheduling of reproduction. Furthermore, hormonal mechanisms are subject to information about the external and internal environment of the individual. Within an evolutionary radiation, the same hormone subsets often regulate the schedules of development as well as adult reproduction and related activities and, moreover, are involved in mechanisms of senescence. We propose that the pleiotropic and epistatic effects from hormonal and neural mechanisms are an important substratum for life history evolution. This analysis of hormonal mechanisms in senescence implicates a role for antagonistic pleiotropy in selection for particular subsets of hormonal mechanisms that can be traced throughout prematurational and postmaturational stages. In the example of the vertebrate MHC (major histocompatibility complex), polymorphic loci have been assembled with pleiotropic actions on several regulatory axes affecting reproduction and other fitness components. We argue that the MHC and other complex loci may be considered as life history gene complexes, with pleiotropic influences throughout the lifespan. While analyses of this kind might suggest that life histories could be evolutionarily rigid, in our interpretation the population genetics that is involved provides a theoretical basis for great flexibility in hormonal regulation during life history evolution. It is possible that life history evolution among taxonomic groups may sometimes be chaotic, which would frustrate strong inferences by the comparative method in the study of life histories between taxonomic groups.