Life span and evolution

Why do many species have a limited life span that vary from species to species? We are all programed to weaken and die. I can only guess that this has some sort of evolutionary advantage that came about because of competition. Still, why do some species have a longer life span than man? It seems like a prey species should have a longer life span than a predator species to me.

I'm hoping someone on this forum knows more about this than I do. Can somebody make sense of this for me?

Originally posted by Metal Brain
We are all programed to weaken and die.

Extremely few living things any are specifically programed by evolution to specifically weaken and die and we are certainly not so programmed. That isn't how evolution works. There is generally no gene or genetic trait specifically evolved to turn on at the latter stage of life with the specific function to make the individual weaken and die unless, for that particular species and in relatively rare cases (and I assume extremely rarely if ever), weakening and dying somehow increases the chances of that individual's genes being passed onto the next generation. They are programmed to maximize the chances of passing on their genes onto the next generation, and that doesn't necessarily mean living longer. Evolution is not about individual survival, which is superfluous from the evolution perspective. Organisms generally evolve to maximize the probability of reproductive success even where and when it is at the expense of individual survival and/or longevity.

A gene for any trait that increases longevity at the expense of reproductive success would be selected against, not for, by natural selection. And a gene for any trait that increases longevity but neither increases nor decreases the probability of reproductive success wouldn't be either selected by nor against natural selection BUT would tend to be lost from the gene pool anyway via genetic drift.

This is part but not the whole explanation of why, for example, people haven't evolved to be able to live hundreds of years long after they have lost the ability to reproduce; we have evolved to pass on our genes, not live for as long as possible. The reason why we weaken and die of old age is because, once we have had our main chance to pass on our genes, we are 'dumped' by evolution, meaning, evolution has made no special provision to make us live longer (or shorter, for that matter) once our main opportunity of reproductive success has come and passed. One why of thinking about that is that evolution simply 'doesn't care' what happens to us as individuals one way or the other once our individual main chance to pass on our genes has come and passed.

Originally posted by Metal Brain
Why do many species have a limited life span that vary from species to species?

Its a large topic that probably cannot be done full justice here. But ultimately it comes down to the most efficient way to reproduce in the niche that an organism is in.
1. It is essential that an organism reaches reproductive age, or assist in the reproduction of siblings or close relatives.
2. How long it lives after first reproductive age depends on many factors to do with the advantages / disadvantages of longevity.
With humans, life is dangerous and in the past life expectancy was in the 30 - 50 range. This meant that most people died from fighting, disease, hunger, accident, rather than old age. There was no evolutionary requirement or advantage to maintain health beyond 50 and so any genes that would benefit old age were not being selected for.

In general DNA copying is not perfect, so older individuals have more mutations than younger ones. There is significant advantage to reproducing as young as possible. So there is an evolutionary trend towards reproduce as young as reasonable, produce enough offspring to replace the population (including considering death rates) and that's it.

This is different when there is reduced cost to having offspring such as is the case for many plants or animals that do not care for their offspring. There are also differences between species that compete for resources within the species vs those that mostly do not.

In summary, its complicated.

Originally posted by Metal Brain
Why do many species have a limited life span that vary from species to species? We are all programed to weaken and die. I can only guess that this has some sort of evolutionary advantage that came about because of competition. Still, why do some species have a longer life span than man? It seems like a prey species should have a longer life span than a p ...[text shortened]... g someone on this forum knows more about this than I do. Can somebody make sense of this for me?

This is outside my area of knowledge, but I think it's connected with how stable the genome is. Cells in our bodies occasionally mutate into cancer cells, to guard against this cells that aren't stem cells can only divide so many times before their chromosomes start to unravel. So for cancer to occur a cancer cell has to overcome this limitation. The balance is between having an individual live long enough to reproduce and raise its offspring and it dieing of cancer before this. Where that balance lies depends on the species life chances. There is no reason for longevity to increase if individuals reproduce quickly anyway, but if reproduction is a relatively rare event then individuals who live longer tend to have more offspring, so one would expect longevity to increase. I don't know if this is true, but one might expect solitary creatures like tigers (up to 26 years according to Wikipedia) to have a longer life expectancy than pack animals like lions (around 20 years).

Originally posted by Metal Brain
Why do many species have a limited life span that vary from species to species? We are all programed to weaken and die. I can only guess that this has some sort of evolutionary advantage that came about because of competition. Still, why do some species have a longer life span than man? It seems like a prey species should have a longer life span than a p ...[text shortened]... g someone on this forum knows more about this than I do. Can somebody make sense of this for me?

Death prevents the older generation from competing with offspring and therefore speeds evolution by removing old genetic combinations.

Originally posted by AThousandYoung
Death prevents the older generation from competing with offspring and therefore speeds evolution by removing old genetic combinations.

Natural selection, being a blind process, wouldn't select against genes that tend to make some individuals compete against some other individual's offspring although if an individual competes with its own offspring then natural selection would select against that but by selecting genes which tend to make the individual behave in such a way as to not to compete against their own offspring but rather help their own offspring rather than dying as, once you are dead, you would be unable to care for your own offspring to improve their chances of reproductive success. So that wouldn't explain why we die of old age.

Also, natural selection, being a blind process, doesn't discriminate between gene combinations that are old and gene combinations that are more recent. It only discriminate between gene combinations that have different tendencies to either aid or hinder the passing on of the genes and doesn't cure how old or new the genes are.

Also, natural selection being a blind process, cannot look ahead long term and do something for the specific function of speeding itself up in the long term (or do anything for any long term function for that matter) . Sexual reproduction evolved because it can give a relatively short-term adaptive ability that can be short-term enough to be naturally selected and the fact that sexual reproduction also speeds up evolution long term is just an accidental side effect of that rather than something which evolution drives to do in particular.

Originally posted by humy
Extremely few living things any are specifically ...

misedit;
That should have been;
"Extremely few living things if any are specifically ...

Originally posted by humy
Natural selection, being a blind process, wouldn't select against genes that tend to make some individuals compete against some other individual's offspring although if an individual competes with its own offspring then natural selection would select against that but by selecting genes which tend to make the individual behave in such a way as to not to compete ...[text shortened]... o improve their chances of reproductive success. So that wouldn't explain why we die of old age.

I don't fully follow your post, but you appear to be arguing against the useful (but wrong) characterisation of evolution planning what its doing. I don't think anyone is actually arguing that.
I also think you are forgetting inter group dynamics. A group that evolves quicker will out-perform a group that doesn't thus evolution does select for fast reproduction cycles. It doesn't require planning ahead.
There is a constant tension between many different factors some of which encourage longer livelihood and some of which encourage shorter. The perfect balance is different for every niche and every species thus we see a wide range.

Originally posted by DeepThought
This is outside my area of knowledge, but I think it's connected with how stable the genome is. Cells in our bodies occasionally mutate into cancer cells, to guard against this cells that aren't stem cells can only divide so many times before their chromosomes start to unravel.

I don't think that is entirely the case. Animals that require long lives for their particular lifestyle evolve mechanisms to deal with cancer. We have not evolved such mechanisms or have lost them because the onset of cancer was typically beyond our life expectancy. Cancers which affect the young do exist but they are rare because we evolve to deal with them. In general though cancer is a threat and a cost to ageing so all else being equal it is better to have shorter lives so there is something to your idea.

Originally posted by humy
Sexual reproduction evolved because it can give a relatively short-term adaptive ability that can be short-term enough to be naturally selected and the fact that sexual reproduction also speeds up evolution long term is just an accidental side effect of that rather than something which evolution drives to do in particular.

Sexual reproduction appeared by accident but it is maintained because it allows mix and match and gene exchange far more effectively than any other mechanism. For large multicelular life it is practically essential.

Originally posted by twhitehead

A group that evolves quicker will out-perform a group that doesn't thus evolution does select for fast reproduction cycles.

then why did large animals such as, say, elephants, evolve so large that the effect of their large size has on their physiology makes them have to have much slower reproduction cycles than that of their smaller ancestors probably had?
Why didn't groups of their ancestors that had faster reproductive cycles always out-perform (because they evolve quicker) than any that didn't thus continually preventing any evolving to have the much slower reproduction cycles that we observe today that we see in animals such as elephants?
Certainly, if evolution always selects for fast reproduction cycles then the slow reproductive cycles of elephants would have never evolved so there must be times when natural selection continually selects for characteristics that happen to result in slower reproduction cycles else animals like elephants couldn't have evolve.

Originally posted by humy
then why did large animals such as, say, elephants, evolve so large that the effect of their large size has on their physiology makes them have to have much slower reproduction cycles than that of their smaller ancestors probably had?
Why didn't groups of their ancestors that had faster reproductive cycles always out-perform (because they evolve quicker) than ...[text shortened]... happen to result in slower reproduction cycles else animals like elephants couldn't have evolve.

What if the selected trait of largeness is prioritized over speedy reproduction?

Originally posted by humy
then why did large animals such as, say, elephants, evolve so large that the effect of their large size has on their physiology makes them have to have much slower reproduction cycles than that of their smaller ancestors probably had?

Because size advantage is more important in that case.

Certainly, if evolution always selects for fast reproduction cycles then the slow reproductive cycles of elephants would have never evolved so there must be times when natural selection continually selects for characteristics that happen to result in slower reproduction cycles else animals like elephants couldn't have evolve.
Evolution is selecting across a large number of factors all of which are competing. Faster reproduction cycles win out over slower ones ALL ELSE BEING EQUAL. Every trait has costs and benefits and every trait is in a cost benefit competition with other traits. For elephants for example, one might say 'bigger is better' as that reduces predation. But it comes at a cost. Thus elephants will evolve to a particular size and then stop getting bigger as the costs (including the requirement for longer lifespans) get to big. But it is a mistake to say that because they got big there is no advantage to being small or having shorter lifespans. It is just that the advantages of being big outweighed the advantages of being small.

The most fascinating part of evolution is when you get traits that only work in a fraction of the population.

Originally posted by twhitehead
Because size advantage is more important in that case.

[b]Certainly, if evolution always selects for fast reproduction cycles then the slow reproductive cycles of elephants would have never evolved so there must be times when natural selection continually selects for characteristics that happen to result in slower reproduction cycles else animals like ...[text shortened]... lifespans. It is just that the advantages of being big outweighed the advantages of being small.

I see now what you were saying.
I am in agreement.