Demographic Homeostasis and the Evolution of Senescence

Existing theories for the evolution of aging and death treat senescence as a side-effect of strong selection for fertility. These theories are well-developed mathematically, but fit poorly with emerging experimental data. The data suggest that aging is an adaptation, selected for its own sake. But aging contributes only negatively to fitness of the individual. What kind of population model would permit aging to emerge as a population-level adaptation? I explore the thesis that population dynamics is inherently chaotic, and that aging is selected for its role in smoothing demographic fluctuations. The logistic equation provides a natural vehicle for this model because it has played a central role in two sciences: Population growth in a resource-limited niche has long been modeled by the differential LE; and, as a difference equation, the LE is a canonical example of the emergence of chaos. Suppose that feedback about depleted resources generally arrives too late to avoid a wave of unsupportable population growth; then logistic population dynamics is subject to chaotic fluctuations. It is my thesis that aging is an evolutionary adaptation selected for its stabilizing effect on chaotic population dynamics.