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Switch to Forum Live View Why every species has a different life span: Answered!
6 years ago  ::  Mar 09, 2008 - 1:51AM #1
papdu
Posts: 337
The reason why all species have evolved and are evolving different life spans is quite simple. Many speculate we die of old age to make room for the next generation, I believe this to be FALSE.

All things being equal, those with the longest life span will have the most children. So, having genes that make me live for 10 Million years are very beneficial, I could have 100,000's of children in that time, much more than folks who die of old age at 100yrs. I and all that inherit my genes for a 10M yr life span will dominate in the population fairly rapidly. The gene for 10Mil lifespan will dominate the world human population. The population of the world will not get out of hand as you may think, other causes of death will keep the population in check to match what the environment can withstain, with respect to human population.

SO, why haven't we OR all life forms evolved an infinite lifespan. The answer is self evident and you are probably thinking it as you read this. NOBODY will live that long anyway as we will all succumb to some form of accidental death long before that, such as murder, predation(including by non-genetic disease- viruses), starvation, lightning, drowning, an accident etc. etc. etc. You’re going to run out of luck sometime along the way as you are still vulnerable to being killed. Though not all life forms have the same statistical probability of being killed, do to their position as a species within their particular environment.

One example is the "mouse" max aprox 2yrs old  as compared to "humans" max aprox 120yrs old lifespan.  Although mice may live up to two years in the lab, the average mouse in the wild lives only about 5 months, primarily due to heavy predation. Let's say we did a hypothetical test and changed the genes of 1 Million wild mice so they may live and stay young for 10 Million yrs with no chance of any genetic diseases to kill it. As long as they don't get killed they will live for the allotted 10Mil yrs, and we did the same with 1 Million humans. The mouse being close to the bottom of the food chain and so small in size probably faces and nearly escapes getting killed by a predator almost every day of it's life, thus they can only go so long before running out of luck as most mice get killed around 5 months in the wild way before old age arrives. Even with the 10 Million yr gene, not a single mouse will ever use those genes as the probability of getting killed is within 5 months or so and the chances of being lucky beyond that, just makes it less and less probable to make beyond that 5, 6, 7 months. They couldn't last 10 yrs not to mention 10 Million yrs.  Same with humans. We being on the top of the food chain die from accidents or predation far less often then the mouse, therefore we are able to capitalize on genes that make us live longer. Yet we have our limits as well, just not as short as the mice....Trees have the longest life span. They also get killed the most in-frequently as they get killed occasionally by strong wind or lightning and perhaps a virus, other than that the probability of living long with out accidental death is far greater for trees then most animals. So they too capitalize on  the low probability of getting killed by evolving a longer life span.
The bottom line is there is no advantage to have genes for living much beyond the probability of accidental death as having genes for living beyond that would just be extra baggage, thus are weeded out.

It’s not that there is a purpose for all species to have certain life span, it’s that there is no purpose/advantage to having genes that make you live beyond your statistical probability of getting killed by accident/predation/starvation etc. That probability is different for every species.

Let me make one more point. Has anyone seen the 1970's movie "LOGAN'S RUN"?  Well if you haven't it takes place sometime in the future, when humans decide for whatever reason,  that all people are to be executed when they turn 30 years of age. I think they did this so humans didn't have to suffer the effects of aging. Well everyone pretty much accepted the culture and actually celebrated the time of there execution....For a moment let's step outside the story of logan's run and lets say humans actually did this.  Since all humans would be executed at 30 yrs old , that would mean there is no longer any DIS-ADVANTAGE to having genetic diseases that show up after 30 years of age and like wise there would be no ADVANTAGE to anyone born with  genes for no genetic disease or perfectly healthy longevity genes for living beyond 30yrs…Folks with genes for cancer that strikes at 35 yrs will not have any less children on average then folks without NO gene for cancer at 35, since everyone gets killed at 30 anyway.......Since no one will reproduce after 30 yrs anyway genes for genetic diseases occurring after the age of 30 will no longer be weeded out and will start accumulating in the human population. Eventually maybe after millions of years even if a humans stopped this tradition of execution at 30 they would probably die right after his/her 30th birthday of rapid old age as perhaps salmon do. This being  a result of millions of years of accumulated genetic diseases within the human population after the age of 30 years old due to the culture of execution at 30 yrs old.

Also it's been noted that animals in captivity live a few years longer into old age then do there wild brethren as they are protected from predation and starvation. HUMANS as well live into there old age, meaning we too are living in sort of captivity(civilization) as compared to our wild brethren of 50 or 100 thousand years ago. We still have longevity genes of our hunter gather ancestors. I will argue since humans are living into there old age in our current environment we must have a lower probability of being killed then our hunter gatherer ancestors. We can now take advantage of genes that makes us live longer.
I believe we are currently evolving much longer life spans to match our current agricultural/city way of life. Anyone who is born with longer healthier longevity genes, that make them stay younger longer and die older, can now capitalize on those genes by having more children, then people with genes for shorter life spans, as the probability of an individual getting killed is much lower then  for our hunter gatherer ancestors. What our new  probability of years getting killed at, is I don’t know, maybe 500, 600 yrs or more.

Also genes for the age of reproductive maturity will evolve accordingly with the newly evolved life span and other variables depending on the species.
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6 years ago  ::  Mar 09, 2008 - 2:36PM #2
Oncomintrain
Posts: 3,044
This is an interesting idea. But I'm not sure I agree.

Recent research in geriatrics indicated our genes actually program us to shut down at a certain point. They don't simply cease to protect us from the effects of aging, they actually actively CAUSE the effects of aging. In other words, sexually reproducing organisms are programmed with planned obsolescence. And there is a good reason for that.

Most populations (humans notably excepted) tend to remain more or less stable for long periods of time. Births roughly equal deaths, and each new generation replaces the one before it. This is a good thing, evolutionarily speaking, because each new generation contains novel mutations and genetic combinations, which keeps the species genetically labile. So on a species level, the offspring are always more important than the parents.

So the older generation needs to stay around just long enough to replace themselves. And then, they need to get out of the way, so they aren't using up resources better used by their offspring (to create yet another generation, etc).

In any species where this rule is violated, you would see a steady accumulation of older generations, competing with the youngest generation for resources. The species' gene pool would stagnate, and genetic lability would suffer. As a result, the species would be less able to adapt rapidly to changing conditions, and the species would eventually die off.

Humans are, of course, a notable exception to the above, on the grounds that our population is anything BUT stable. So I suppose we could develop toward longer and longer lifespans. I suspect though, that this will have at least as much to do with medical science as it has to do with biological evolution. Indeed, human beings have more or less taken themselves out of natural selection, so evolution doesn't act especially strongly on us.

Interesting related tidbit: I once read that every warmblooded species has roughly the same number of lifetime heartbeats. That is to say if you multiply the average heartrate of the species by the average lifespan, this number is more or less a constant, across all warm-blooded species. The one glaring exception to this rule? Human beings. I don't know if that has any significance, but I found it intriguing.
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6 years ago  ::  Mar 09, 2008 - 3:53PM #3
papdu
Posts: 337
"""So the older generation needs to stay around just long enough to replace themselves. And then, they need to get out of the way, so they aren't using up resources better used by their offspring (to create yet another generation, etc)."""

I agree, but disagree. I still say we evolved the life span for that reason I've stated--around the statistical probability of getting killed. All the things you have mentioned may all be true as well. I will argue that since all life has a life span due to that probability, all the other charactreistics followed in evolutionary adaption, making everything else about the organism perfectly adapted for that lifespan.. .....One evolutionary adaption causes a chain reaction of the  evolution of other adaptions within the organism.....For instance species with the most protective parents just so happen to have offspring wich are dependant on protective parents for survival. You may think that how could you have one without the other. I would say parents being protecive had it's advantages so evolved,  this caused a chain reaction evolution in children to take advantage of this new system of parenting by not wasting resources on growing so fast ,there no need to as your being taken care of. Genes for learning and growing in childhood evolved within the new system of child raising. Making all look like a pefect fit.

Likewise with aging. Since we evolved lifespan due to the statistical probability of getting killed, it makes  sense for natural selection to evolve the most efficiet way of aging, whatever that may be, for the organism and it's offspring since your gonna die anyway.

Also who populates the next gerneration is unimportant. All things being equal it is more important for me to live 1 Million years and reproduce than for my children to do it as I contain 100% of my genes and my children have only 50%.
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6 years ago  ::  Mar 09, 2008 - 4:07PM #4
mountain_man
Posts: 39,136

OncominTrain wrote:

......Interesting related tidbit: I once read that every warmblooded species has roughly the same number of lifetime heartbeats. That is to say if you multiply the average heartrate of the species by the average lifespan, this number is more or less a constant, across all warm-blooded species. The one glaring exception to this rule? Human beings. I don't know if that has any significance, but I found it intriguing.


That seems to me to be closer to the question of lifespan than the OP. It's not just the number of heart beats but the pace of life in general. Those that move fast, mice for an example, use a higher amount of energy in a shorter amount of time than, say, a tortoise. They put a bigger strain on the body and just burn out quicker. There's an old saying; you can burn out faster than you can rust out.

It is also a good point that prey species, like those mice again, have large litters earlier and more often than those that are not prey species. That makes sense evolutionarily, they have to in order to get some of their offspring to breeding age to carry on the species.

I'd say, in the end, it's probably a combination of the two.

Dave - Just a Man in the Mountains.

I am a Humanist. I believe in a rational philosophy of life, informed by science, inspired by art, and motivated by a desire to do good for its own sake and not by an expectation of a reward or fear of punishment in an afterlife.
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6 years ago  ::  Mar 09, 2008 - 7:49PM #5
papdu
Posts: 337
From part of article at wikipedia.com on Senescence

http://en.wikipedia.org/wiki/Senescence



""""Ageing is believed to have evolved because of the increasingly smaller probability of an organism still being alive at older age, due to predation and accidents, both of which may be random and age-invariant. It is thought that strategies which result in a higher reproductive rate at a young age, but shorter overall lifespan, result in a higher lifetime reproductive success and are therefore favoured by natural selection. Essentially, ageing is therefore the result of investing resources in reproduction, rather than maintenance of the body (the "Disposable Soma" theory), in light of the fact that accidents, predation and disease will eventually kill the organism no matter how much energy is devoted to repair of the body. Various other, or more specific, theories of ageing exist, and are not necessarily mutually exclusive.

The geneticist J. B. S. Haldane wondered why the dominant mutation which causes Huntington's disease remained in the population, why natural selection had not eliminated it. The onset of this neurological disease is (on average) at age 45 and is invariably fatal within 10-20 years. Haldane assumed, probably reasonably, that in human prehistory, few survived until age 45. Since few were alive at older ages and their contribution to the next generation was therefore small relative to the large cohorts of younger age groups, the force of selection against such late-acting deleterious mutations was correspondingly small. However if a mutation affected younger individuals, selection against it would be strong. Therefore, late-acting deleterious mutations could accumulate in populations over evolutionary time through genetic drift. This principle has been proven correct. And it is these later-acting deleterious mutations which are believed to cause, or perhaps more correctly allow, age-related mortality.

Peter Medawar formalised this observation in his mutation accumulation theory of ageing[2] [3]. "The force of natural selection weakens with increasing age — even in a theoretically immortal population, provided only that it is exposed to real hazards of mortality. If a genetic disaster... happens late enough in individual life, its consequences may be completely unimportant". The 'real hazards of mortality' are typically predation, disease and accidents. So, even an immortal population, whose fertility does not decline with time, will have fewer individuals alive in older age groups. This is called 'extrinsic mortality.' Young cohorts, not depleted in numbers yet by extrinsic mortality, contribute far more to the next generation than the few remaining older cohorts, so the force of selection against late-acting deleterious mutations, which only affect these few older individuals, is very weak. The mutations may not be selected against, therefore, and may spread over evolutionary time into the population.

The major testable prediction made by this model is that species which have high extrinsic mortality in nature will age more quickly and have shorter intrinsic lifespans. This is because there is too little time before death occurs by extrinsic causes for the effects of deleterious mutations to be expressed and, therefore, selected against. This is borne out among mammals, the most well studied in terms of life history. There is a correlation among mammals between body size and lifespan, such that larger species live longer than smaller species in controlled/optimum conditions, but there are notable exceptions. For instance, many bats and rodents are similarly sized, yet bats live much, much longer. For instance, the little brown bat, half the size of a mouse, can live 30 years in the wild. A mouse will live 2–3 years even with optimum conditions. The explanation is that bats have fewer predators, and have lower overall metabolic activity, due to lengthier periods of dormancy, so therefore low extrinsic mortality. Thus more individuals survive to later ages so the force of selection against late-acting deleterious mutations is stronger. Fewer late-acting deleterious mutations = slower ageing = longer lifespan. Birds are also warm-blooded and similarly sized to many small mammals, yet live often 5–10 times as long. They clearly have fewer predation pressures compared with ground-dwelling mammals. And seabirds, which generally have the fewest predators of all birds, live longest.

Also, when examining the body-size vs. lifespan relationship, predator mammals tend to have longer lifespans than prey animals in a controlled environment such as a zoo or nature reserve. The explanation for the long lifespans of primates (such as humans, monkeys and apes) relative to body size is that their intelligence and often sociality helps them avoid becoming prey. Being a predator, being smart and working together all reduce extrinsic mortality."""
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6 years ago  ::  Mar 10, 2008 - 2:07PM #6
Erasmus
Posts: 1,400
[QUOTE=papdu;342943]All things being equal, those with the longest life span will have the most children.[/quote]

I can grow more bacteria in a few hours than there have been humans in the entire history of the world.  Obviously, you are very wrong.
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6 years ago  ::  Mar 10, 2008 - 2:33PM #7
papdu
Posts: 337
[QUOTE=Erasmus;346566]I can grow more bacteria in a few hours than there have been humans in the entire history of the world.  Obviously, you are very wrong.[/QUOTE]

Read carefully "ALL THINGS BEING EQUAL"

So I was comparing those of the same species.

So yes an individual bacteria of the same species that has the longer life span than another within it's species will on average have more offspring
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6 years ago  ::  Mar 10, 2008 - 4:57PM #8
Oncomintrain
Posts: 3,044
[QUOTE=papdu;346648]Read carefully "ALL THINGS BEING EQUAL"

So I was comparing those of the same species.

So yes an individual bacteria of the same species that has the longer life span than another within it's species will on average have more offspring[/QUOTE]

But if (she) survives long after his offspring are mature, he will be competing with his own offspring for resources, which is self-defeating, from a genetic perspective. Further, if he lives long enough, he will still be competing with his offspring even as they compete with THEIR offspring, basically making life much harder for the youngest generation, thereby damaging the species' genetic labilty and chances of survival.

So I would guess that evolution would favor an optimal age that balances the benefits of maximizing offspring with the hindrance that the parent's continued survival represents. Rather than trending toward longer and longer lifespans, organisms trend toward a certain central balance-point.

In my opinion...
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6 years ago  ::  Mar 11, 2008 - 2:24AM #9
papdu
Posts: 337
[QUOTE=OncominTrain;347099]But if (she) survives long after his offspring are mature, he will be competing with his own offspring for resources, which is self-defeating, from a genetic perspective. Further, if he lives long enough, he will still be competing with his offspring even as they compete with THEIR offspring, basically making life much harder for the youngest generation, thereby damaging the species' genetic labilty and chances of survival.

So I would guess that evolution would favor an optimal age that balances the benefits of maximizing offspring with the hindrance that the parent's continued survival represents. Rather than trending toward longer and longer lifespans, organisms trend toward a certain central balance-point.

In my opinion...[/QUOTE]

But don't forget that you and your offspring are competeing with folks who are not related or carry your genes as well. Let's say you and your offspring keep sticking around and reproducing more and more causing famine in the world . The thing is, the famine will strike those with short living genes just as equally than those with longevity genes.Famine knoews no difference. If the famine  kills 30% of the population it will equally kill 30% of the short lived gene people and 30% of the longlived people. On average there will be more folks left with the longlived genes due to sticking aroung longer.

It doen't matter that your offspring were affected by the famine and some died. Whats important is that more of your offspring were around after, then folks with the shorterlifespan genes. And they would be.

Look at this way. You need to point out the mechanism which will decrease your ability to have more offspring then someone born with genes for a shorter life span, so that shorter lifspan will increase in the population. I get what your getting at, but I still can't visualize it. When my grandhather was 90 years old I saw nothing in the envoirnoment that increased his chances of dying over me who was 28 at the time other than his old age. I assume if he didn't die of old age he would still be alive today. It's true folkes who live for ever take up more space in the world but there also taking up space from those who don't carry their genes....I suppose you feel very lucky you weren' born with genes that kept you at 20 year olds body and the lifespan of 500 yrs. Because according to you since living long will take up space and cause you to compete with your offspring, natural selection somhow will cause you to die before those with lifspans of only 70 yrs old. Natural selection isn't about need it's about what will happen in the moment. You keep telling me why it would be bad , but your not showing the mechanism by wich you will be killed off. Picture yourself living 500 yrs what will kill you off more offen then those who live at  100. I see nothing targeting you over someone born with 100 yr genes . 

Another thing. You must agree that humans evolved from organisms with shorter lifespans, an ape, monkeys, small mammals living at the feet of dinosaurs. We have been evolving longer life spans. How do you know this has stopped.

Evolution has nothing to do with need. The anscestors of man didn't evolve upright walking because they needed to in order to survive on the open plains. They evolved upright walking because the first human anscestor chose to stand on two legs much more offen then they would usaully do in the jungle. Theory is that the ape that man evolved  from was as smart as achimp. When they were approached by predators they most likely chose to group together, grab the nearest rock or branch and wildly screaming throw them at predators while standing on two legs.   This caused natural selection to select for those better at  using this strategy....The anscestors of Baboons also left the trees and began living on the open plains, but the baboon anscestor chose to run from predators and if closed in on try to fight and bite the predator, causing natural selection to select those that were more successful at that startegy.
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