Inevitable senescence? : contributions to evolutionary-demographic theory
The main thrust of my dissertation is to understand whether and when senescence is an inherent characteristic of life. Hamilton (1966) claimed to have proven that "senescence is an inevitable outcome of evolution". One major result of my work is that no dogmatic statement can be made about the universality of senescence. By carefully studying Hamilton's paper on the moulding of senescence, I show that Hamilton did not prove that senescence "cannot be avoided by any conceivable organism". I have developed simple models that contribute general insights to evolution ary demographic theory. The models are designed to shed light on whether and when non-senescent life-history strategies could be optimal. All models show that senescence is not inevitable. Sustenance can be an optimal life-history strategy. The results of my size-based models suggest that species with the ca pability of continued growth after the onset of reproduction are candidates for non-senescence. The results of my vitality-based model suggest that the costs of growth and maintenance and, to an almost equal extent, the costs of reproduction are major determinants of the choice between senescence and sustenance. My dissertation can be viewed as a theoretical exploration of the inter-species diversity of aging, i.e., of how varied aging can be for different species and of what factors determine whether a species' strategy involves sustenance or senescence. My models suggest that a remarkable variety of patterns may be optimal under different circumstances. The limited empirical data available suggests that species may show a rich diversity of age-schedules of mortality, fertility and growth. This dissertation shows that senescence and sustenance are two complemen tary sides of the process of aging. One cannot be deeply understood without the other. The new, burning question that arises from my work is: In what kind of species does senescence evolve and in what kind of species is it sustenance that evolves?