There had been no evolutionary theory of aging in the 50 years since August Weismann had disavowed his own. \u00a0Medawar was astute enough to realize that Weismann was correct to seek an evolutionary understanding of aging. \u00a0Aging cannot be understood from thermodynamics or physical processes of attrition. \u00a0No physical law requires aging. \u00a0Medawar was also correct in judging that Weismann\u2019s proposed solution was no solution at all. \u00a0\u201cWeismann caters twice round the perimeter of a vicious circle. \u00a0By assuming that the elders of his race are decrepit and worn out, he assumes all but a fraction of what he has set himself to prove.\u201d<\/p>\n
Medawar proposed the theory that natural selection can only work on living, reproducing individuals. \u00a0But in the wild, there are so many hazards that can lead to death that, past a certain age, there are very few remaining alive. \u00a0In nature, everyone dies before they reach old age. \u00a0This creates a \u201cselection shadow\u201d. \u00a0Bodies are evolved to be healthy, strong or fertile up to the age where there are still survivors in nature. \u00a0But at advanced ages, natural selection has never had an opportunity to work her magic, so we should not be surprised that the organism is ill-adapted and falls apart.<\/p>\n
We get old and die because of evolutionary neglect. \u00a0Natural selection needs living, reproducing individuals to select from, or it is ineffectual; hence we expect that aging takes over and the body deteriorates soon after that age at which predators and disease and other hazards of the wild have thinned the population near to zero.<\/p>\n
The rest is history<\/b><\/p>\n
Very quickly, Medawar\u2019s idea was enshrined in the canon of evolutionary theory. \u00a0Building on Medawar, two more ideas were added. \u00a0One was the concept of \u201cmutational load\u201d. \u00a0If there was no natural selection at work for \u201clate-acting genes\u201d, then random mutations would creep in, and this would account for the organism going to pot. \u00a0This became the Mutation Accumulation<\/a> theory. \u00a0The other was the idea that selection at late ages may be weak but not zero, and it could then be overpowered by the drive to maximize fertility early in life, even if it had bad consequences for fitness later on. \u00a0This became the Antagonistic Pleiotropy<\/a> theory.<\/p>\n The idea that, in the wild, no one lives long enough to die of old age made a great deal of intuitive sense. \u00a0George Williams (of the pleiotropy theory) added a refinement: that the early stages of senescence would likely have consequences\u00a0for individual competitiveness, so he based his theory on the idea that selection against aging was weak but not zero. \u00a0But everyone was agreed that the fitness consequences of aging were very slight, if not actually negligible.<\/p>\n Evolutionary theory went on to develop on this basis, and continued to be embellished for 40 years.<\/p>\n Fact check<\/b><\/p>\n The inconvenient truth came to light gradually, and several decades on. \u00a0In physics and chemistry, experimental science is an attractive calling because practitioners get to hang out in a lab and perform magic with nifty apparatus and spiffy electronics. \u00a0But experimental ecology is a field science requiring travel to remote locations, and many lonely, patient hours of observation, away from the comforts of home. \u00a0The work is often left to doctoral students who are in no position to protest.<\/p>\n So it was 25 more years before evidence started to accumulate that could bear on the question, how many animals in the wild are dying from the (early) effects of aging?<\/p>\n In principle, it is not hard to determine an answer. \u00a0Collect carcasses in the woods and use established forensic techniques to estimate the age of the animal when it died. \u00a0Once you have enough cases, you can plot a curve: \u00a0how many deaths? (on the y axis) vs what age? (on the x axis). \u00a0For example:<\/p>\n How to interpret the results? \u00a0If there were no aging in the wild, then we must expect that the percentage of individuals dying at every age is the same. \u00a0But the number remaining gets smaller and smaller, so the absolute number of deaths would go down with age. \u00a0The math tells you that \u201cno aging\u201d corresponds to a falling exponential curve.<\/p>\n If the number that we actually find is flat with age, or even if it declines with age but not as rapidly as an exponential curve, this is evidence that aging is taking a toll on fitness in the wild.<\/p>\n It was 1991 before Daniel Promislow<\/a> first collected and interpreted the appropriate statistics for 56 different mammals<\/a> in the wild. \u00a0He was a doctoral student at Oxford, and this study launched his career. \u00a0In 46 of the 56 species, he found an increasing risk of death with age. \u00a0In several species, data were complete enough that he was able to detect a Gompertz curve<\/a>, meaning that for a given individual, risk of death climbs exponentially with age.<\/p>\n The Gompertz shape of the mortality curve had been known for 150 years\u2014nothing new there. \u00a0The surprise was that previous to Promislow, scientists had thought that the Gompertz shape only applies in protected environments, like humans in civilization and animals in zoos. \u00a0In the wild, it was expected that (according to Medawar) everyone dies too early to see the rising shape of the Gompertz curve.<\/p>\n The significance of these results was not lost on Promislow. \u00a0He boldly asserted that his results conflicted with the accepted evolutionary theories for aging. \u00a0He was also modest and tactful enough to allow for reasons that his conclusion might be premature, and that adjustments could be made to permit the evolutionary theories to hold their own.<\/p>\n The Evidence Piles on<\/b><\/p>\n In 1998, Robert Ricklefs<\/a> expanded on Promislow\u2019s results by including more mammal surveys and some birds. \u00a0Ironically, he titled his piece Confirmation of a Fundamental Prediction<\/a>, but in fact the results made all the extant theories for evolution of aging quite untenable. \u00a0He fitted mortality curves for each of the species in the study, and reported parameters from these curves. \u00a0From these data, it is a small further step to answer the question, What proportion of deaths in species can be ascribed to aging? \u00a0Ricklefs set up the equations and provided all the parameters, but he never completed the calculation. \u00a0Later, I filled in those numbers, the \u201cpercentage of senescent deaths\u201d for each species. \u00a0You can read them in the column highlighted in orange.<\/p>\n As you can see, there are no animals for which the impact of senescence in the wild is negligible. \u00a0Many are clusted in the range 15-30%. \u00a0Some are over 70% \u2014 meaning, roughly, aging is reducing fitness in these species by more than 2\/3.<\/p>\n Heroic field work<\/b><\/p>\n All the above work is based on field studies, data compiled after the fact through searching for remains. \u00a0But the cleanest kind of study would be an experiment, planned in advance, where individual animals could be tracked in the wild and their fates determined by direct observation. \u00a0As a Canadian grad student in the early 2000s, Russell Bonduriansky<\/a> set himself the daunting task of individually labeling, releasing, and recapturing thousands of antler flies to answer directly, how did their risk of death change with age? \u00a0His doctoral work was stunning enough to be profiled in Nature<\/a>. \u00a0The result: 28% of antler fly deaths were due to aging.<\/p>\n <\/p>\n Nature doesn\u2019t care if you die once your fertility is gone<\/b><\/p>\n The numbers above present a dilemma for evolutionary theory. \u00a0Scientists dealing loosely with this question sometimes say, animals die once their fertility is ended. \u00a0It\u2019s no surprise that evolution has permitted these animals to age and die once they have reproduced and replaced themselves. \u00a0But the theory can\u2019t escape so easily, for two reasons.<\/p>\n First, this doesn\u2019t answer the question, but only kicks the can down the road. \u00a0Yes, there is no selective advantage to be gained by continuing to live on past the end of fertility. \u00a0But why should fertility decline in the first place? \u00a0Why haven\u2019t we all adopted the growth pattern of trees and lobsters, continuing to grow and produce more offspring with each passing year? \u00a0(Or, if there are size constraints that keep land animals from growing forever, at least we should be maintaining our fertility, not losing it with age.)<\/p>\n Second, responding to the argument about \u201conce they have replaced themselves\u201d… \u00a0We should note that this is a flat denial of the dominant \u201cselfish gene\u201d view of evolution. \u00a0In standard evolutionary theory, there is no such thing as \u201cenough\u201d, because individuals are in an arms race to dominate the next generation with their genes. \u00a0If I have 6 offspring and you have 7, it will not be very many generations before my descendants are completely crowded out by yours (according to the way standard evolutionary calculations are performed). \u00a0This perspective highlights the evolutionary paradox that natural selection has tolerated declining fertility and increasing mortality in so many different animal species.<\/p>\n The bottom line<\/b><\/p>\n Sixty years after Medawar, it is untenable to maintain that aging exists in a \u201cselection shadow\u201d. \u00a0The negative consequence of aging for individual fitness is a force to be reckoned with. \u00a0But theorists have yet to face this particular monster. \u00a0I still hear Medawar\u2019s hypothesis cited as gospel regularly in papers and at conferences. \u00a0The disconnect between theory and observation is stark.<\/p>\n Two footnotes<\/b><\/p>\n![](\"https:\/\/www.crowcanyon.org\/ResearchReports\/sitetesting\/Fig_finl\/Chp_19\/Fig19_01.gif\")
<\/p>\n![\"ExponentialDecline\"](\"https:\/\/scienceblog.com\/wp-content\/uploads\/sites\/2\/2016\/05\/ExponentialDecline.gif\")
<\/p>\n![\"Promislow91\"](\"https:\/\/scienceblog.com\/wp-content\/uploads\/sites\/2\/2016\/05\/Promislow91.png\")
<\/p>\n![\"Ricklefs98-table\"](\"https:\/\/scienceblog.com\/wp-content\/uploads\/sites\/2\/2016\/05\/Ricklefs98-table.png\")
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