Imagine a world in which senescence is eliminated, so that death rates do not increase with age but remain throughout life at the level for eighteen-year-olds, that is, about one per thousand per year. Some people would still die at all ages, but half the population would live to age 693, and more than 13 percent would live to age 2000!" -- Nesse and Williams (1994) Why we get sick: the new science of Darwinian medicine
Although Darwin's ideas are increasingly influential
(at least among scientists), Darwin himself is dead. In a world without senescence, he might still be alive. In The dawn of Darwinian medicine.
Q. Rev. Biol. 66, 1-22 (1991), Williams and Nesse offered the standard evolutionary explanation for aging:
Because the force of natural selection is stronger at earlier ages to which larger numbers survive, a gene that causes substantial morbidity and mortality during the tail end of the expected life span in the wild may nonetheless be favored if it has even minor earlier benefits.
The most important of these "earlier benefits" appears to be more reproduction earlier in life. One of my students came up with some interesting ideas about the implications of this tradeoff between reproduction and longevity, which I will discuss here once our paper is published.