HORMONES AND THE PHYSIOLOGICAL ARCHITECTURE OF LIFE-HISTORY EVOLUTION

Hormones play key roles in the regulation of animal and plant life his tories, particularly in the timing of transitions between prematuratio nal stages and in the scheduling of reproduction. Furthermore, hormona l mechanisms are subject to information about the external and interna l environment of the the individual. Within an evolutionary radiation, the same hormone subsets often regulate the schedules of development as well as adult reproduction and related activities end, moreover, ar e involved in mechanisms of senescence. We propose that the pleiotropi c and epistatic effects from hormonal and neural mechanisms are an imp ortant substratum for life history evolution. This analysis of hormona l mechanisms in senescence implicates a role for antagonistic pleiotro py 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 sever al regulatory axes affecting reproduction and other fitness components . We argue that the MHC and other complex loci may be considered as li fe history gene complexes, with pleiotropic influences throughout the lifespan. While analyses of this kind might suggest that life historie s could be evolutionarily rigid in our interpretation the population g enetics that is involved Provides a theoretical basis for great flexib ility in hormonal regulation during life history evolution. It is poss ible 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.