Shattered: Medawar's test tubes and their enduring legacy of chaos

Med Hypotheses. 2000 Feb;54(2):326-39. doi: 10.1054/mehy.1999.0856.


Medawar's 1952 paper 'An Unsolved Problem of Biology' underlies most subsequent theoretical work regarding the evolution of aging; it concludes that aging is accidental and could not have evolved; this prevents reconciling the growing body of evidence suggesting the existence of multiple, evolved, aging systems. The paper features a well-known thought experiment using test tubes to show why aging could not evolve. Medawar assumes that constant, random, breakage sufficiently represents lethal forces of nature; however, famine, drought, predation, disease, and accidents each uniquely affect populations. Predation is the only evolving force that continually invents new ways to kill members of the prey populations; thus all prey defenses to predation will eventually be defeated. Defenses to non-evolving or non-obligate lethal forces, however, should quickly evolve. Thus unevolving, identical test tubes cannot adequately represent biological populations. The example also ignores population booms and busts which often occur in nature. By ignoring these issues, Medawar examines only one population age distribution skewed towards younger individuals in predator-dominated environments while ignoring predator-free populations skewed towards older individuals after population crashes. Further, Medawar's test tubes lack meaningful competition for finite resources, and ignore declining fertility which occurs in all aging species. Medawar concludes that older individuals are too few in number to influence the population's gene pool for or against aging. This conclusion is found to be incorrect when variations in the age of reproductive senescence are introduced into a predator-free population.A new thought experiment with competing strains of algae corrects for these issues and shows that aging evolved and is retained so that groups retain enough genetic variability to allow for rapid evolution of a defense to novel predation. The example shows reasons why the rate of aging is directly linked to the reproductive rate, litter size, metabolic rate, reproductive senescence, and fixed body size. It also suggests that in the absence of predation, immortality would quickly evolve if not for the evolution of highly-conserved aging systems. Prior analysis of aging evolution is incorrect due to theorists' rejection of the idea of group selection. It is believed to be 'impossible' to select for mutations that are bad for the individual but good for the group. However, mutations that are neutral to young individuals which are only deleterious if expressed at older ages can accumulate in early-mortality, predator-dominated environments. Removing the predator allows deleterious mutation expression. Positive group selection then occurs amongst traits that are negative to the individual. Further, group selection is a universal force that occurs between local, non-breeding groups and not, as theorists propose, between distant groups of potentially interbreeding species. Local survivors migrate to replace extinct, related species. The antagonistic pleiotropy theory, which was created to salvage the idea of accidental aging, is examined and shown to be untenable. The hypothetical antagonistic pleiotropy genes that are beneficial to young while detrimental to old individuals, predicted to exist in the 1950s, are unlikely to exist, have not, and likely will not be found in sufficient quantity to participate in the aging process.

MeSH terms

  • Aging / physiology*
  • Animals
  • Humans
  • Models, Biological*
  • Nonlinear Dynamics*