What is lifespan regulation and why does it exist?

The development of a unified conceptual framework for the field of biogerontology has been impeded by confusing and misleading terminology. Thus, distinctions and definitions are provided for key terms (and their concepts) used in the paper: senescence, lifespan, potential lifespan, essential lifespan, and lifespan regulation. An organismal perspective is then used to examine the relationships between reproduction, lifespan regulation and senescence. The principal conclusions drawn from this examination are: (1) the inevitability of death makes physiological investments in reproduction a higher priority than somatic maintenance, (2) the race between reproduction and death creates a probabilistic window of time (essential lifespan) within which reproduction must occur, (3) the integrated network of genetic processes responsible for achieving essential lifespan (lifespan regulation) must be evolutionarily conserved and extensively regulated, (4) senescence is a stochastic byproduct of these regulated processes rather than a direct target of natural selection, and (5) genomic instability (an important stochastic component of senescence) plays no active role in lifespan regulation.