21 October 2021
Aging
Wear and tear theory
DEF: Aging is the result of fundamental limitations, inevitable small random changes that accumulate with time (stochastic theories of aging); a tendency of all matter to decay into a more random, less ordered state (entropy).
source: http://www.programmed-aging.org
Wear and tear theories of aging - problems
- a great variety of life-span in living organisms even if their size, activity and metabolism are controled
- entropy can be reversed using energy; extensive maintenance and repair capability of organisms
Non-programmed aging
Mutation accumulation MA (Medewar 1952)
exponential decline -> half-life x reproduction -> differential reproductive effect of cohorts -> evolutionary importance of individuals decreases with age (deterious mutations accumulated with time are not sweep out by selection due to low reproductive effect of the older cohorts)
Medawar, P.B, An Unsolved Problem of Biology., 1952. H.K. Lewis & Co., London.
MA and seelction
Fabian D, Flatt T (2011) The Evolution of Aging. Nature Education Knowledge 3(10):9
Mutation accumulation MA - emipiric data
Amyloid hypothesis and Alzheimer disease
Makin S (2018) Alzhiemer’s on trial. Nature 559
Mutation accumulation MA - problems
a great variation of organisms life-span (even if controlled for various confounding variables); the model organism as put by Medewar does not exist
a number of diverse organisms (e.g. salmon, octopus, marsupial mouse) display instances of death closely following an act of sexual reproduction
More criticisms:
Makin S (2018) Alzhiemer’s on trial. Nature 559
Bowles J (2000) Shattered: Medawar’s test tubes and their enduring legacy of chaos. Med Hypotheses 54:326–339
Antagonistic pleiotropy AP (Williams 1957)
Pleiotropy: a situation in which a single gene controls more than one trait. Thus, aging is caused by the combined effect of many pleiotropic genes that each had a beneficial effect in an animal’s youth but also had an adverse side effect in older age.
Prediction: Species with younger age of sexual maturity and more vigorous reproduction traits would tend to have shorter life spans.
Austad SN, Hoffman JM (2018) Is antagonistic pleiotropy ubiquitous in aging biology? Evol Med Public Heal 2018:287-294
Antagonistic pleiotropy AP
Problems:
- Ross experiment - the laboratory line of flies selected for longevity (lived twice as long as the the short-lived line) also laid more eggs than the short-lived flies.
- Why this unavoidable side effects issue would only affect maintenance of the condition of an adult organisms when tasks have to be performed in the development, growth, and day-to-day operation of an organism?
MA and AP hypotheses - comparison
Fabian D, Flatt T (2011) The Evolution of Aging. Nature Education Knowledge 3(10):9
Disposable soma DS (Kirkwood 1977)
DEF: organisms age due to evolutionary trade-offs between growth, reproduction, and DNA repair maintenance
Kirkwood TBL (2002) Evolution of Aging. Mechanisms of Aging and Development 123:737-745
Disposable soma DS - problems
- resource allocation - evidence against
Shanley DP, Kirkwood TBL (2000) Calorie restriction and aging: A life-history analysis. Evolution 54:740-750.
- simultanous reproduction and ageing process
Evolution of longevity (Cichoń 1997)
Cichoń M (1997) Evolution of longevity through optimal resource allocation. Proc R Soc B Biol Sci 264:1383-1388
Programmed aging
Group/Kin Selection
DEF:benefit to survival of a group may offset some degree of individual fitness disadvantage and thus allow for the evolution of organism characteristics that are adverse to individual fitness such as programmed aging
A lot of criticisms…
But see grandmother hypothesis