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Science Forum Index » Bio Evolution Forum » The ultimate cause of aging
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| dkomo |
 Posted: Fri Mar 07, 2008 8:36 am |
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Don't get too excited. I'm using the term "ultimate" here in the sense
of "evolutionary":
http://en.wikipedia.org/wiki/Proximate_and_ultimate_causation
The terms "ultimate" and "proximate" causation have been
enthusiastically adopted by scientists who study human and animal
behavior/psychology. But these terms were originally coined by Ernst
Mayr and meant to be applied to evolutionary biology as a whole.
"The basic reason why survival and reproduction decline with age is that
selection acts more weakly on later ages. Imagine an organism that does
not senesce, that is, that maintains itself with the same rate of
survival and reproduction indefinitely. It will still suffer accidental
mortality and so, on average, reproducing early will produce more
offspring than deferring reproduction until later; death might strike
first. Therefore, natural selection acts more strongly on variations
that act early in life and acts more and more weakly on late-acting
acting variations (see Box 20.2). If, as a result, senescence starts to
evolve, then there will be a feedback, so that selection on later ages
becomes even weaker. In the extreme case, where an organism such as a
salmon reproduces only once, there is absolutely no selection on
subsequent survival."
Nicholas Barton et al, _Evolution_, p. 562
As an example, suppose our physiologically immortal animal is called the
shmoo, and it has a death rate of 10% per year due to environmental
causes such as predation, lack of food or water, parasites, diseases,
heat, cold, floods, severe snowstorms, or accidents such as falling into
swiftly moving rivers or from cliffs. That means that the average
schmoo has only a 35% chance of living past the age of 10, and a 12%
chance of living past 20. Suppose further that once a shmoo reaches
maturity at the age of 5, it reproduces each year with undiminished
vigor until it dies.
Now suppose the shmoo gets a mutation called the "death bomb" which is
dormant until the age of 25, then gets expressed and kills the unlucky
bearer within a matter of days. The evolutionary theory of aging
predicts that there will be little negative selection against the death
bomb gene because if a shmoo reaches the age of 25, which in itself is
rather unlikely, it has already reproduced numerous times. The death
bomb has little effect on the shmoo's fitness, hence is effectively a
neutral mutation. However, it can become fixed in the shmoo population
through genetic drift.
Importantly, another consequence of the evolutionary theory of aging is
the following:
"Based on this evolutionary explanation of aging, we predict that in
organisms with a low rate of accidental mortality, senescence should be
less severe. This is consistent with the relatively long life of birds,
bats and turtles, who suffer less predation because they can fly or have
thick shells. Across species of birds and mammals, the rate of
senescence is indeed correlated with baseline mortality (Fig. 20.12).
Nicholas Barton et al, _Evolution_, p. 562
Fig 20.12 is a log-log scatter plot of the rate of senescence plotted
against baseline mortality for many different wild birds, zoo birds, and
mammals. It indeed shows a fairly tight correlation between rate of
senescence and accidental mortality rate.
How does this theory explain why a dog at the age of 10 experiences many
of the same symptoms of old age as a human at 80 years -- arthritis,
cancer, blindness, extreme fatigue, etc.
Well, dogs are descended from wolves, and wolves tend to live in pretty
harsh environments. Not only that, but they have to fear being attacked
and possibly killed by other members of the pack in the struggle for
dominance. Therefore, probably not many wolves even reach the age of
ten. The domestic dog has apparently inherited the aging history of the
wolf.
Interestingly, tens of thousands of years of human dog breeding, both
accidental and deliberate, has done little to improve the longevity of
the domestic dog. In fact, some breeds have shortened life spans due to
inbreeding and possessing oddball physical traits that humans like.
My guess is that dog longevity hasn't increased much despite the rather
cushy life styles dogs have by being with humans because humans have
fairly tight control over dog reproduction, and they don't often allow
aged dogs to breed. Again, there is no strong selection for genes that
increase canine longevity.
But how do you explain the fact that the dog seems to age 8 times faster
than the human if aging is simply parts wearing out? Do the dog's
"parts" wear out 8 times faster than the human's? Why? Aren't the
dog's "parts" pretty much the same as the human's? The same proteins,
cells, organs, and so on?
Here is where the "ultimate" explanation of aging is lacking. In order
to answer these questions we have to go into the "proximate"
explanations of aging involving physiology, biochemistry, cell biology,
molecular biology, and so forth. There are dozens if not hundreds of
proximate aging processes involved, many of which still await discovery.
However, I think it is quite encouraging that such an eightfold
difference in aging rate between dogs and humans can exist. That shows
what we can potentially accomplish in slowing aging once we gain control
over the proximate mechanisms involved. Also, consider the fact that
some tortoises can live 200 years and bristle cone pines can live 5000
years. There seems to be plenty of "headroom" in increasing our own
longevity. If natural selection can control plant and animal life spans
over such enormous ranges, then so can we once we develop the
appropriate technology.
Finally, I don't think that the evolutionary theory of aging explains
human life span very well. Throughout the hundred thousand year history
of modern humans there have been people who have lived into their 80's
and 90's despite the fact that the great majority of people throughout
the millions of years of evolution from hominid to human seldom lived
much past the age of 30. It seems to be too great a disparity to be
explained by this theory.
--dkomo@cris.com |
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| John W Edser |
| Posted: Fri Mar 07, 2008 8:18 pm |
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dkomo <dkomo871@comcast.net> wrote:-
Quote: "The basic reason why survival and reproduction decline with age is that
selection acts more weakly on later ages. Imagine an organism that does
not senesce, that is, that maintains itself with the same rate of
survival and reproduction indefinitely. It will still suffer accidental
mortality and so, on average, reproducing early will produce more
offspring than deferring reproduction until later; death might strike
first.
JE:-
I have been arguing this within sbe for about 10 years. The net result is
that survival cannot be a fitness maximand.
Quote: Therefore, natural selection acts more strongly on variations
that act early in life and acts more and more weakly on late-acting
acting variations (see Box 20.2).
JE:-
Natural selection must act to maximise fitness, no matter how fitness
happens to be defined. Since only fertile forms can possibly pass on their
genes, selection _for _ can only act on later developed fertile forms.
Therefore selection operates less strongly "on variations that act early in
life" and can only act fully after all immature forms have been raised to
fertile adulthood. Selection against (not for) these forms acts if they die
before they could be raised to fertile adulthood. This provides each
selectee with just the ONE Total Darwinian Fitness (TDF) as a falsifiable
fitness maximand per population. TDF is defined as: the total number of
strictly fertile forms raised to adulthood per parent per population. If two
or more maximands are provided per selectee per population (as polycentric
Neo Darwinism allows) then they must contest until only one survives OR they
fitness mutualize as entirely independent fitnesses, i.e. at least two
independent selectees must now be hypothesised.
Quote: If, as a result, senescence starts to
evolve, then there will be a feedback, so that selection on later ages
becomes even weaker. In the extreme case, where an organism such as a
salmon reproduces only once, there is absolutely no selection on
subsequent survival."
Nicholas Barton et al, _Evolution_, p. 562
JE:-
The human gap between giving birth and raising babies to fertile adulthood
is about 15 years. This would mean that a woman at menopause would be
selected to have at least another 15 years to complete the job allowing her
to live to 50+15 =65. She may be selected to live longer if she provides a
mutualised fitness benefit by doing do. An example would be that older
people become the repository of tribally based knowledge.
Quote: As an example, suppose our physiologically immortal animal is called the
shmoo, and it has a death rate of 10% per year due to environmental
causes such as predation, lack of food or water, parasites, diseases,
heat, cold, floods, severe snowstorms, or accidents such as falling into
swiftly moving rivers or from cliffs. That means that the average
schmoo has only a 35% chance of living past the age of 10, and a 12%
chance of living past 20. Suppose further that once a shmoo reaches
maturity at the age of 5, it reproduces each year with undiminished
vigor until it dies.
JE:-
Yes, it is entirely possible to argue that reproduction and death can evolve
a fixed survival rate. The most famous example are salmon. They are selected
to die by their own hand after spawning entirely within the ecosystem in
which they have given birth because this will increase and not decrease the
chance of raising more of their own infertiles to fertile adulthood.
Attempting to swim away and do it all over again mostly resulted in the
parents dying somewhere else, donating their bodies to a foreign ecosystem.
It seems to me that ecosystem concept hinges on mutualised fitnesses acting
across the ecosystem. Without any TDF understanding it remains impossible to
produce a mutualised fitness concept because no frame of reference exists to
allow it. This is why I keep on stressing that Hamiltonian altruism is not
at all real. Inclusive fitness has no constant frame of reference so it is
only a mathematical, i.e. not at all a scientific argument.
Regards,
John Edser
Independent Researcher
edser@ozemail.com.au |
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| Tim Tyler |
| Posted: Fri Mar 07, 2008 8:18 pm |
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Guest
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dkomo wrote:
Quote: "The basic reason why survival and reproduction decline with age is that
selection acts more weakly on later ages. Imagine an organism that does
not senesce, that is, that maintains itself with the same rate of
survival and reproduction indefinitely. It will still suffer accidental
mortality and so, on average, reproducing early will produce more
offspring than deferring reproduction until later; death might strike
first. Therefore, natural selection acts more strongly on variations
that act early in life and acts more and more weakly on late-acting
acting variations (see Box 20.2). If, as a result, senescence starts to
evolve, then there will be a feedback, so that selection on later ages
becomes even weaker. In the extreme case, where an organism such as a
salmon reproduces only once, there is absolutely no selection on
subsequent survival."
Nicholas Barton et al, _Evolution_, p. 562
That's one theory. However, aging has many causes - e.g. see:
http://en.wikipedia.org/wiki/Reliability_theory_of_aging_and_longevity
--
__________
|im |yler http://timtyler.org/ tim@tt1lock.org Remove lock to reply. |
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| Lorentz |
| Posted: Fri Mar 07, 2008 8:18 pm |
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On Mar 7, 1:36 pm, dkomo <dkomo...@comcast.net> wrote:
Quote:
But how do you explain the fact that the dog seems to age 8 times faster
than the human if aging is simply parts wearing out? Do the dog's
"parts" wear out 8 times faster than the human's? Why? Aren't the
dog's "parts" pretty much the same as the human's? The same proteins,
cells, organs, and so on?
I read some analysis of the evolution of dogs, but I don't
remember the reference. However, I do remember a little bit relevant
to your discussion.
Both dogs and humans have evolved to keep certain juvenile traits
longer than individuals in their closest relatives (i.e., other
species in the same family or order). However, they have evolved by
different mechanisms.
Human beings are neotonous. Their developmental aging process has
slowed down, even including the development of their sexual organs.
They reach adolescence much later, and grow old much later. Their
aging has especially slowed down with regards to the ability to learn,
which is in most species of mammals basically a juvenile trait.
Neotony is characteristic of animals which undergo a large amount of
competition, in environments where the abiotic environment is rather
benign (Gould, Ontogeny and Phylogeny). Humans have to compete against
each other, the savagery of the weather is somewhat secondary.
The main juvenile trait in humans is the ability to learn, by
which I mean the maximum rate of learning. A mammal brain stops
growing by adolescence, so if the adolescence is delayed the brain
grows larger and the ability to learn gets larger. An adult human
learns at a significant fraction of the rate of an infant human,
whereas an adult chimpanzee learns at a rate that doesn't compare to
that of an infant chimpanzee. Even though the infant chimpanzee and
the infant human learn at approximately the same rate, adult humans
learn at a much faster rate than an adult chimpanzee.
Humans also live longer than a chimpanzee. The longer lifespan is
a secondary effect of the neotony. The aging process was slowed down
so that the intelligence can last longer. Of course, there is a little
feedback. A person who continues to learn all his life becomes more
knowledgeable as he grows older. So the ability to learn is greatly
enhanced by the delay of senescence. So the ratio of births rate to
death rate may increase with age in primitive human societies. An old
human knows how to avoid leapards better than an adolescent. A chimp
wouldn't benefit as much from a slowed down aging process. His chances
of being eaten by a leapard are just as great as an adult as for an
adolescent.
A dog is paedomorphic. Its aging process is speeded up. His sexual
organs age faster than his brains. A mammal brain stops growing by
adolescence, so if the adolescence is advanced the brain doesn't grow
as large and the ability to learn gets smaller.
Paedomorphy is characteristic of animals which undergo a large a
small amount of competition, in environments where the abiotic
environment is rather hostile (Gould, Ontogeny and Phylogeny). Sort of
field of bullets selection. The earliest dogs were wolves that lived
in the caveman equivalent of junkyards. There was lots of food thrown
away in garbage heaps, so the wolves didn't have to compete with each
other. The abiotic environment may have been hostile, people like to
put their garbage heaps in areas that no one would want to live. Like
areas with no water, etc. When it rained, there was probably enough
water for all the dogs, when there was a drought, they all died.
Being smart wasn't as important as getting along with the humans
who lived near by, not bothering them. Being
Dogs don't live as long as wolves. However, compared to wolves
the dog is a sexual god. Dogs reach complete sexual maturity within
two years, wolves take longer. Wolves are also much smarter than dogs,
at least as adults.
Quote:
Here is where the "ultimate" explanation of aging is lacking. In order
to answer these questions we have to go into the "proximate"
explanations of aging involving physiology, biochemistry, cell biology,
molecular biology, and so forth. There are dozens if not hundreds of
proximate aging processes involved, many of which still await discovery.
The adrenal glands may be involved in neotony. I know that in the
axolytol, a neotonous salamander, the adrenal glands are slowed down.
The axolytyl grows into a superlarge tadpole and reproduces that way.
I guess the competition in it little Mexican lake caused neotony. Add
iodine, and it metamorphizes. |
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