EE - Evolution and Aging
I am sorry that I only just found this thread, I should have been reading this from the begining, but better late than never. I haven't read all of the posts yet, but I did notice some posts on this topic which I certainly have to read. I will do so, but it will take time.
As far as I knew, the most prevalent theory of 'why' we age has been covered reasonably well. I know of at least 3 papers which essentially cover the details of it, and which in my mind do so very well in a complimentary way. The three papers are:
1. Medawar PB. 1952. An Unsolved Problem of Biology. Lewis, London
2. Williams GC. 1957. Pleiotropy, Natural Selection, and the Evolution of Senescence. Evolution 11:398-411
3. Kirkwood TBL. 1977. Evolution of Aging. Nature 270:301-304 and/or
Kirkwood TBL. 2002. Evolution of Ageing. Mechanisms of Ageing and Development 123:737-745
Medawar proposes that age is a result of a declining selective pressure as an organism 'ages' (ie: as time passes). Williams then takes it a step further and connects the less vigourous actions of Selection with Pleiotropy, proposing that genes which confer an early advantage but a later disadvantage will still be selected. These later side-effects will be dealt with by selection later as required. Kirkwood then builds on this by proposing his 'Disposable Soma Theory' which says more of the same, but from a different sort of perspective.
In my opinion, Williams hit the nail on the head in 1957. I am posting this partially hoping to be corrected on this point, but since it hasn't happened yet, I firmly believe that Williams' theory explains why we age perfectly, and now all we need to do is apply its logic to the 'how' and solve it.
What Williams Said:
First of all, it makes no sense of all to propose that biological organisms 'decay' over time: they are not static objects, they are dynamic systems in constant exchange with their environment. More importantly, they all produce themselves from morphogenetic processes: Any system which can form itself can surely maintain itself.
Assuming a world where there was no such thing as senescence, death would be determined by accident, predation, starvation etc. As time passes the probability of succumbing to one of these causes of death accumulates. Eventually you would die. For example: In a population of critters in this theoretical world, a graph of their percentage of survival over time would decline as periodically one and then another and then another dies from one incident or another. Evolution would have a keen interest in making their reproductive age earlier, or somehow making the organism more robust so as to survive to reproductive age. Evolution can do little about making them all immune to accident, starvation etc though.
"This would produce a decline in reproductive probability, because the probability of reproduction at any age is a function of the probability of surviving to that age. No matter how low the mortality rate there is always a greater chance of surviving to age A than to age A + 1." (Williams 1957)
Individual traits may act at distinct times/conditions/places etc. Any genetic trait which acts early in life while have a profound affect, affect every organsm which is born with it. A trait which acts late in life (whether late be weeks, months, years, or decades in the human case) will have little affect since it may only be expressed in a small proportion of organisms which carry it, the rest being eaten, starving, falling etc. Where a gene is Pleiotropic (ie: Has more than one effect depending on its situation, conditions, timing etc), and one affect acts early, and another acts later, the early acting gene has more evolutionary weight to it than the late acting gene. ie: If the early gene is good, and the late gene is bad, it will most likely still be a gene that will be 'selected FOR'.
"Natural Selection may be said to be biased in favour of youth over old age whenever a conflict of interests arises."
(Williams 1957)
Any organism which has a longer period of reproductive potential would of course be selected for over an equivalent organism which only has a breif window of reproductive potential: It is therefore beneficial for evolution to act to reduce the latter negative effects.
"Senescnce might be regarded as a group of adaptively unfavourable morphogenetic changes that were brought in as side effects of otherwise favourable genes, and which have only been partly expurgated by further selection." (WIllimas 1957)
And that is essentially the theory. Reading the paper itself is well worth it. I beleive that Oxidative Damage theory of aging is a perfect example of antagonistic Pleiotropy, The advantage of being an Aerobic organism beign huge, but the gradual accumulation of damage to important structures being a long term negative. Over time evolution has fought to restrict and stop this damage by using DNA repairs mechanisms, proteins recyclers, free radical scavengers etc. It has not been able to stop the damage, but it has dulled it down to a background level that allows us a life span perfectly suited to our pre-historic lifestyle.
