Developments in general evolutionary theory and quantitative population genetics during the early twentieth century set the stage for theorizing on the evolution of senescence. Medawar (1946) developed the “wear-and-tear” theory based on observations that with age organisms have an increasing likelihood of dying due to intrinsic (belonging to the real nature of a thing; not dependent on external circumstances; inherent) factors (reviewed in Chapter 1). He illustrated this point using the now classic example of test tubes in a laboratory (Austad 1992). Test tubes do not senesce; still, through time, they tend to break due to laboratory mishap and intrinsic fragility. If most test tubes break (die) within 2 years, a change (mutation) that affects test tubes only after 2 years will have little effect on longevity in the overall population. However, a change that improves a 1-year-old test tube's survival, or, better yet, that of a 6-month-old tube, will have a substantial effect on both average and maximum time until loss in that test tube population. Medawar further noted that with age selection pressure declines in concert with decreasing fertility in natural populations, ultimately becoming minimal or non-existent.
Two conflicting views typify evolutionary theories on senescence. Most early theorists saw senescence as the outcome of a specific genetic program designed by natural selection to eliminate unneeded post-reproductive individuals. A minority of gerontologists now subscribe to various permutations of this genetic programming theory (see Clark 1999 for a recent review).