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Author Topic:   Discussing Evolutionary Theories that Explain Aging
CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 1 of 24 (692060)
02-26-2013 6:02 PM


This is a nature article discussing these theories.
http://www.nature.com/...ary/the-evolution-of-aging-23651151
The Mutation Accumulation Hypothesis
Following the logic outlined above, Medawar (1946, 1952) reasoned that, if the effects of a deleterious mutation were restricted to late ages, when reproduction has largely stopped and future survival is unlikely, carriers of the negative mutation would have already passed it on to the next generation before the negative late-life effects would become apparent. In such a situation, natural selection would be weak and inefficient at eliminating such a mutation, and over evolutionary time such effectively neutral mutations would accumulate in the population by genetic drift, which in turn would lead to the evolution of aging. This is known as Medawar's mutation accumulation (MA) hypothesis (Figure 3A). The effects of such a mutation accumulation process would only become manifest at the organismal level after the environment changes such that individuals experience less extrinsic mortality (e.g., due to decreased predation) and thus live to an age where they actually express the symptoms of aging.
The Antagonistic Pleiotropy Hypothesis If it is true that selection cannot counteract deleterious effects at old age, he argued, then mutations or alleles might exist that have opposite, pleiotropic effects at different ages: genetic variants that on the one hand exhibit beneficial effects on fitness early in life, when selection is strong, but that on the other hand have deleterious effects late in life, when selection is already weak. This idea is known today as the antagonistic pleiotropy (AP) hypothesis for the evolution of aging (see Rose 1991, Flatt & Promislow 2007, Figure 3B). Williams pointed out that, if the beneficial effects of such mutations early in life outweigh their deleterious effects at advanced age, such genetic variants would be favored and enriched in a population, thus leading to the evolution of aging. Thus, under Williams' hypothesis, the evolution of aging can be seen as a maladaptive byproduct of selection for survival and reproduction during youth.
A fundamental corollary of Williams’ AP hypothesis is that early fitness components such as reproduction should genetically trade-off with late fitness components such as survival at old age, so that, for example, genotypes with high early fecundity should be shorter lived than those with low reproduction (e.g., Williams 1957, Rose 1991, Charlesworth 1994, Hughes & Reynolds 2005). In a somewhat similar vein, Kirkwood’s 1977 "disposable soma" (DS) hypothesis predicts that the optimal level of investment into somatic maintenance and repair will evolve to be below that required for indefinite survival. The idea here is that the evolution of a higher investment is unlikely to pay off since the return from such an investment may never be realized due to extrinsic mortality. Moreover, investment into reproduction — or early fitness components in general — might withdraw limited resources that could otherwise be used for somatic maintenance and repair. Such resource allocation trade-offs can thus been seen as a physiological extension of Williams' AP model.
Although the relative frequency of MA versus AP is still debated (both may typically go hand in hand - see also Moorad & Promislow 2009), there is robust evidence today for the existence of fitness trade-offs that are consistent with the notion of AP (for a recent discussion of the positive evidence see Flatt & Promislow 2007, and Flatt 2011, but also see Moorad & Promislow 2009). Whether such trade-offs are physiologically caused by competitive energy or resource allocation — as would be expected under the DS hypothesis — remains somewhat controversial, but the trade-offs themselves are well established (see Flatt 2011). Most importantly, the kinds of trade-offs postulated by Williams, have been found at the evolutionary level: for example, fruit flies that were artificially selected for increased late-life reproductive success were found to be long-lived at the expense of reduced early fecundity in several, now classical, experiments in the labs of Michael Rose and Leo Luckinbill (Rose & Charlesworth 1980, Rose 1984, Luckinbill et al. 1984). These elegant experiments represent the first solid empirical tests of the evolutionary theory of aging (Rose 1991).
The classical evolutionary theory of aging has therefore two fundamental cornerstones: MA and AP. However, it is worth noting that both models are conceptually very similar: under MA, aging evolves through the accumulation of effectively neutral mutations with deleterious late-life effects, whereas, under AP, aging occurs due to mutations with beneficial early- and deleterious late-life effects. In reality, probably both types of mutations occur in populations, yet their relative frequencies remain unknown. Furthermore, the age distribution of mutational effects may be much more complicated than these two scenarios suggest (e.g., Moorad & Promislow 2008).
ps: I still have problems with quotes. I didnt write any of that.
Edited by CoolBeans, : No reason given.

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Admin
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Message 2 of 24 (692061)
02-27-2013 7:50 AM
Reply to: Message 1 by CoolBeans
02-26-2013 6:02 PM


Hi CoolBeans,
To quote a passage just put [qs] at the front and [/qs] at the back.
Quoted passages should be brief. Please describe what you'd like to discuss in your own words. If there's a tie-in to the creation/evolution debate then please describe that, too.
AbE: And please correct the spelling of "Discussing" and "Aging" in the title.
Edited by Admin, : AbE.
Edited by Admin, : Another AbE.
Edited by Admin, : Typo.

--Percy
EvC Forum Director

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CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 3 of 24 (692062)
02-27-2013 3:19 PM
Reply to: Message 2 by Admin
02-27-2013 7:50 AM


The thing is that aging doesnt bring any benefits so its paradoxical to natural selection, or at least it seems to be the case.

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CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 4 of 24 (692063)
02-27-2013 3:30 PM
Reply to: Message 3 by CoolBeans
02-27-2013 3:19 PM


Though I suspect that it might benefit the species by giving more opportunity to their offspring.

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Admin
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Message 5 of 24 (692065)
02-27-2013 3:35 PM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Discussing Evolutionary Theories that Explain Aging thread in the Proposed New Topics forum.

  
Genomicus
Member (Idle past 1941 days)
Posts: 852
Joined: 02-15-2012


Message 6 of 24 (692071)
02-27-2013 4:25 PM
Reply to: Message 3 by CoolBeans
02-27-2013 3:19 PM


The thing is that aging doesnt bring any benefits so its paradoxical to natural selection, or at least it seems to be the case.
I'm fairly well-versed in the biology of aging, and so, with all due respect, I have to ask you this: did you actually read and understand what you posted with regards to antagonistic pleiotropy? Antagonistic pleiotropy and the hyperfunction theory of aging provide an elegant and simple model for understanding the biology of aging. There is nothing really paradoxical about it.
Edited by Genomicus, : No reason given.

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Taq
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Joined: 03-06-2009
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Message 7 of 24 (692084)
02-27-2013 5:12 PM


Social Species
It would seem to me that there are also strategies involving social cooperation or quick adaptation through genetic changes.
In social species the cooperation of older generations improves the fitness of younger generations and encourages the proliferation of genes that support the ability to contribute to intraspecies cooperation later in life. If a grandparent is able to protect a grandchild and help them have children of their own then these traits may be passed on at a higher rate.
Compare this to the exact opposite strategy where grandparents are actually competing with grandchildren for limited resources. In this case, removal of the grandparents from the population results in stronger selection of new traits and may actually help speed up the process of adaptation.

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CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 8 of 24 (692106)
02-27-2013 6:37 PM
Reply to: Message 7 by Taq
02-27-2013 5:12 PM


Re: Social Species
Its similar to what I said. I think that it helps species as a whole and not in individuals. I would like to know of their are counterarguments to this.
Edited by CoolBeans, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 9 of 24 (692110)
02-27-2013 6:58 PM
Reply to: Message 3 by CoolBeans
02-27-2013 3:19 PM


The thing is that aging doesnt bring any benefits so its paradoxical to natural selection, or at least it seems to be the case.
Well, the point is that everything's going to die anyway.
There's a story about Henry Ford. It seems he told his employees to look through junkyards at junked Model Ts and report on why they were broken. When they came back, he observed in their reports that there was one part that was never broken, no matter how beat-up the rest of the car was. So Ford went to his engineers and said: "You're using too much steel in that part, make it weaker."
If you're going to die of starvation, disease, or being eaten by a sabre-toothed tiger anyway, then there's no point in natural selection making you immune to aging, because a variation which would make you resistant to old age after you're dead has no selective advantage. And a variation which makes you more likely to die after you're dead is selectively neutral. And such a variation which also makes you fitter and healthier while you're alive would be favored by natural selection.

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CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 10 of 24 (692118)
02-27-2013 7:39 PM
Reply to: Message 9 by Dr Adequate
02-27-2013 6:58 PM


No, thats not it. Its about what selective advantage would aging give.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 11 of 24 (692123)
02-27-2013 7:54 PM
Reply to: Message 10 by CoolBeans
02-27-2013 7:39 PM


No, thats not it. Its about what selective advantage would aging give.
But it doesn't have to give an advantage to be permitted by the theory. It can be neutral, or it can be part of a trade-off in which it is not advantageous per se.

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Genomicus
Member (Idle past 1941 days)
Posts: 852
Joined: 02-15-2012


Message 12 of 24 (692134)
02-27-2013 10:22 PM
Reply to: Message 9 by Dr Adequate
02-27-2013 6:58 PM


Well, the point is that everything's going to die anyway.
I think the question that CoolBeans is posing here is why we age biologically. The issue isn't really why we die; it's why we undergo senescence, and how this can be reconciled with natural selection.
Aging (i.e., the gradual decay of the organism's components) makes an organism more prone to death by non-biological means (e.g., predators). So, at first sight, it might seem like a real challenge to natural selection/evolutionary theory: wouldn't populations that possess more mechanisms in place to halt aging be more likely to survive than populations that do not? This question is answered by antagonistic pleiotropy and the hyperfunction theory.

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NoNukes
Inactive Member


Message 13 of 24 (692174)
02-28-2013 1:09 PM
Reply to: Message 12 by Genomicus
02-27-2013 10:22 PM


wouldn't populations that possess more mechanisms in place to halt aging be more likely to survive than populations that do not?
Why would we expect the answer to be yes unless we had an extremely naive understanding?
Allowing old critters to hang around indefinitely and to continue having offspring actually slows down the development of new types. The result would be species with less adaptability. We get all of the benefit we need by having a reasonably long siring/birthing period in a finite lifetime.

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nwr
Member
Posts: 6408
From: Geneva, Illinois
Joined: 08-08-2005
Member Rating: 5.1


Message 14 of 24 (692181)
02-28-2013 3:56 PM
Reply to: Message 3 by CoolBeans
02-27-2013 3:19 PM


The thing is that aging doesnt bring any benefits so its paradoxical to natural selection, or at least it seems to be the case.
Of course it brings benefits.
You cannot have long term persistent memory without aging. The way to avoid aging, is for tissue to be continuously renewed. And continuous renewal also implies a continuous erasing of memories.

Fundamentalism - the anti-American, anti-Christian branch of American Christianity

This message is a reply to:
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CoolBeans
Member (Idle past 3614 days)
Posts: 196
From: Honduras
Joined: 02-11-2013


Message 15 of 24 (692198)
02-28-2013 10:18 PM
Reply to: Message 14 by nwr
02-28-2013 3:56 PM


How does it affect memory? Why couldnt we have a mechanism to change that?
I think I may have an answer to the last question but Im just checking.

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