New models for new perspectives in the biology of senescence.

This essay presents examples of life history patterns that are not often discussed. Life spans range nearly a million-fold between different species of higher animals and plants and must be species characteristics under considerable genetic control. A comparative approach to senescence reveals a vast variety in temporal organization, both among species as well as between and within populations that may vary over as large ranges of scale and qualitative characteristics as do morphological and biochemical variations. Species comparisons across many levels of biological organization involving the life histories of many species besides the usual few mammals, insects, and nematodes importantly expand the view of mechanisms that limit life spans. The presumption that age-correlated changes are all adverse to some degree and that most components of an organism should decay as the life span is approached is reevaluated.