Aging Theory

Questions and Answers

Theodore C. Goldsmith


This set of questions and answers concerning biological aging theories is a companion document for the book: Introduction to Biological Aging Theory.

What is biological aging (senescence)?

  Senescence refers to the internally-caused gradual deterioration and eventual death seen in mammals and most animals.


What is meant by lifespan?

Average lifespan refers to the average time an individual member of a species could be expected to live in the absence of any external limitations on lifetime such as predators, infectious diseases, food supply, or habitat availability. Maximum lifespan is the maximum observed lifespan for a species, such as under zoo conditions. Mammal lifespans vary over a range of more than 200:1.


 What is an age-related disease?

An age-related disease or condition is one in which incidence and severity drastically increase with age such as cancer, heart disease, stroke, Alzheimer’s disease, cataracts, hearing loss, etc. These diseases have many different causes and different treatments directed at those causes have been developed and applied. Age-related diseases and conditions are similar between mammal species but occur on vastly different age schedules. Aging theories attempt to explain these observations.


What is an evolutionary mechanics theory?

  An evolutionary mechanics theory attempts to describe the evolution process, the “how it works” aspect of evolution.


What is Darwin’s survival-of-the-fittest or natural selection evolutionary mechanics theory?

Darwin suggested in 1859 that the evolution process caused organisms to acquire evolved inheritable design characteristics (traits) that caused individual members of a species possessing the trait to produce more adult descendants than those not possessing the trait. Darwin’s concept plausibly fits the vast majority of observed traits and is widely thought to be a comprehensive explanation for the evolution process.

Why are genetics discoveries important to evolutionary mechanics theories?

 Genetics discoveries have exposed issues with details of Darwin's evolutionary mechanics concept and suggest that the evolution process is more complex and time consuming than previously thought. These discoveries act to increase the feasibility of changes to Darwin's mechanics concepts.

Are there current scientific disagreements regarding evolution theory?

There is very little disagreement regarding the fact of evolution. Current species are descended from earlier and different species and the evolution of Earth life has progressively and accumulatively continued for billions of years starting from single-cell organisms. Darwin’s survival-of-the-fittest concept plausibly fits the vast majority of observations concerning organism designs. However, some observations, specifically including biological aging and animal altruism, did not fit leading to multiple proposed modifications to Darwin’s evolutionary mechanics concept.


What is the main current disagreement regarding evolutionary mechanics?

The main current disagreement concerns the degree to which the growth and non-extinction of a species population affects the evolution process relative to the survival and reproduction of  individuals. This issue is crucial to aging theories because there is wide agreement that aging reduces an individual's ability to reproduce, at least in mammals. There are now multiple theories to the effect that aging increases a population’s ability to avoid extinction and grow.


What is an evolutionary theory of aging?

An evolutionary theory of aging attempts to explain how aging relates to the evolution process and more specifically, why senescence varies so much between species and otherwise resembles evolved inherited species-specific characteristics (traits) of organisms.


What is an evolutionary non-programmed theory of aging?

Modern non-programmed (non-adaptive) theories of aging suggest that there are many natural deteriorative processes that act to limit lifespan and that the evolutionary need to combat these different processes decreases with age. Therefore, different species only evolved and retained the internal ability to combat the deteriorative processes to the extent necessary to produce a species-specific lifespan.


What was Peter Medawar’s key evolutionary mechanics concept regarding aging?

Medawar suggested that the force of evolution toward living and reproducing longer under wild conditions declines with age in a species and  population-specific manner. Lack of  internally imposed senescence would provide little benefit to a population beyond the age at which essentially all of the members of a wild population could expect to be dead from external causes such as predation, disease, and starvation.


Why was Medawar’s concept crucial to modern aging theories?

Medawar’s concept (1952) explains why biochemically and physically similar species that presumably have similar exposure to natural deteriorative processes frequently have very different internally determined lifespans.


What is phenotypic linkage and why is it important to evolutionary mechanics theories?

 Most elements of an organism’s design have a relationship with the other elements. Therefore, changing any one element (e.g. femur length) typically requires complementary “linked” changes to other design elements such as muscles, tendons, and blood supply to result in a net fitness benefit. This has implications for the nature of the evolution process, particularly regarding time-scale.


What is a genomic linkage and why is it important?

 The genomic design of an organism can cause otherwise unrelated phenotypic traits to be linked to each other in such a way as to inhibit the evolution of an organism design that contains one of the traits without the other. Multiple aspects of a diploid organism’s genomic design such as pleiotropy, chromosomes, and genetic distance can result in such a genomic linkage.


Why is selective breeding not like evolution?

 Selective breeding is directed at enhancing or attenuating a few organism traits that vary between individuals capable of being interbred with relatively little concern about inadvertent changes to other traits and operates on a very short time scale. Evolution is concerned with the combined net effect of all of an organism’s traits on fitness and because of many inter-trait linkage mechanisms is a much more complex and a very much longer process. Evolution can alter traits that do not vary between individual members of a population because mutations can introduce new variations.


What is the antagonistic pleiotropy theory of aging?

 The antagonistic pleiotropy theory suggests that aging is the result of a genomic linkage between aging and some organism trait that benefits younger organisms. George Williams (1957) proposed that such an antagonistic linkage between a beneficial trait and a somewhat adverse trait such as senescence would prevent the evolution process from overcoming aging despite his observation that aging had some adverse effect on a wild population.


What is pleiotropy?

 Pleiotropy refers to the observation that a single gene can affect more than one phenotypic property. Single-gene genetic diseases can have multiple diverse symptoms. Pleiotropy is one form of genomic linkage.


What is the disposable soma theory of aging?

 The disposable soma theory of aging suggests that because maintenance and repair activities required to sustain life consume significant material and energy resources, a species could trade longer survival and reproduction opportunity in older organisms for increased reproductive activity in younger organisms. This idea is an extension of Medawar’s idea that the evolutionary value of survival and reproduction declines with age. Consequently, the evolution process evolved a design that purposely decreased maintenance and repair in older organisms to allow more resources to be applied to reproduction in younger organisms.


What is the population vs. individual controversy in evolutionary mechanics?

 Darwin’s evolutionary mechanics concept is very individual-oriented: The evolution process selects traits that aid an individual possessing the trait to survive and reproduce better than an individual lacking the trait. Subsequent evolutionary mechanics theories are more population-oriented and consider that the evolution process is driven by characteristics and needs of a population. Evolution operates to increase the chance that a population will avoid extinction. There are still fierce arguments as to which or which combination of theories is correct.

What is group selection?

 Group selection is one of a family of post-1960 theories to the effect that a benefit to the survival of a group of individuals can offset individual disadvantage and cause the evolution of an individually adverse trait like senescence or animal altruism. Variations of this idea (e.g. kin selection, small group selection) vary mainly regarding the size of the population considered. Some programmed aging theories are based on this idea.

What is a programmed aging theory?

 Programmed aging theories suggest that internally limiting individual lifespan beyond a species and population-specific age creates an evolutionary benefit for a population of those individuals and that therefore the evolution process caused the development of biological mechanisms that purposely limit individual lifespan. Aging is genetically programmed like growth or puberty or mating seasons. Many different population benefits from limiting individual lifespan have been proposed.


What is evolvability theory?

 Evolvability theory suggests that an organism’s ability to evolve (i.e. evolvability) can itself be affected by evolved organism design characteristics (traits), and that further, a trait that increases evolvability can be evolved and retained even if it causes some individual disadvantage. An increase in evolvability benefits a population by allowing it to adapt more rapidly or precisely to changes in its external world. Evolvability is one of the post-1960 population-oriented theories.


What is an evolvability-based aging theory?

 Aging theories based on evolvability contend that internally limiting individual lifespan increases evolvability in multiple ways and thus increases a population’s ability to avoid extinction. The first such theory was published by Weismann in 1882.


What is negligible senescence?

 Negligible senescence refers to the observation that some species (e.g. Pacific Rockfish) apparently do not age, that is, do not exhibit measurable age-related decline in fitness parameters like strength, speed, sensory acuity, susceptibility to disease, or reproductive ability.


What is maintenance and repair in connection with aging theories?

 Living organisms have very extensive capabilities to prevent or repair damage from many deteriorative processes such as injury, wear and tear, infections, cell death, etc. Many aspects of senescence appear to be caused by progressive decrease in the effectiveness of maintenance and repair functions. Multiple aging theories incorporate maintenance and repair.


What is the caloric restriction effect?

 The caloric restriction effect refers to the observation that restricting an animal’s access to food often increases lifespan (up to a point), especially in smaller mammals.


What is Hutchinson-Gilford progeria and why is it significant to aging theories?

 Hutchinson-Gilford progeria is a very rare human genetic disease that grossly accelerates many manifestations of aging. This is a clue that there are causes of senescence that are common to many manifestations and thus favors programmed aging theories.


What are the major medical research issues surrounding aging theories?

 The current major issue is whether or not senescence, per se, is a medically alterable and therefore treatable condition. Can pharmaceutical agents and treatment protocols be devised that generally delay senescence and thereby delay the appearance of all or most manifestations of aging including serious highly age-related diseases? Non-programmed theories suggest the answer is “no,” programmed aging theories suggest the answer is “yes.” The different theories also suggest that very different biological mechanisms are ultimately responsible for age-related diseases and conditions and therefore suggest multiple different paths toward treating such diseases.

Further Reading on Aging Theories: 

Aging Theory Info    Programmed Aging Info

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