Is death a product of evolution

Williams (1957)

quote:

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The selective value of a gene depends on
how it affects the total reproductive
probability. Selection of a gene that confers
an advantage at one age and a disadvantage at
another will depend not only on the
magnitudes of the effects themselves, but also
on the times of the effects. An advantage
during the period of maximum reproductive
probability would increase the total
reproductive probability more than a
proportionately similar disadvantage later on
would decrease it. So natural selection will
frequently maximize vigor in youth at the
expense of vigor later on and thereby produce
a declining vigor (senescence) during adult
life. Selection, of course, will act to minimize
the rate of this decline whenever possible.
The rate of senescence shown by any species
will reflect the balance between this direct,
adverse selection of senescence as an
unfavorable character, and the indirect,
favorable selection through the age-related
bias in the selection of pleiotropic genes.

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Nothing much to add to that - but it makes for an interesting read. I'm fairly sure the science has moved forwards a lot from there, but Williams is such an accessible science writer, I thought I'd include his paper in this thread for those interested in it.

The critical part of the OP is that it is talking about dying of 'natural causes' that is to say: of old age. Why do bodies grow old and die? Would it not be evolutionarily 'stable' to live longer and have more children and look after them and have more children and so on and so on?Genes replicate therefore they are - a set of genes that managed to create a machine capable of replicating its genes until it was killed by an enemy/predator/accident etc would have a huge selective advantage over other gene machines that slowly grew old, stopped spreading genes and died.Its a great and valid question to ask of evolution, since a simple look at evolution would have us believe that animals should be living longer and longer - yet we don't!

No - our cells replace themselves so the cells you have at 50 are different than the cells when you are 10. If you kept the same cells all your life you would eventually 'wear down', but that isn't what is going on in biology. Scientific laws are usually formal statements rather than 'things die'. I suppose one could say that there is an empirical law of death. I think it is simpler to refer to fact of death than require some kind of law (because theoretically a life could exist that would never die of old age). One proposed theory discusses it in terms of pleitropy. I referenced the paper in a post just above this one. Actually - no. The population would still have a peak value at which level enough members do not survive long enough to reproduce. This would have the effect of balancing the population size out (we see it today anyway - most organisms do not live long enough to die of old age).Any race of beings that didn't die of old age would either have to exponentially increase its resources to account for the population growth, or it would find its growth cut short. At that point, any genes that provided for a long life would not be selected for (or might even be selected against), and so they would probably lose it.And so - a population would never be in a stable state with no aging process.

I've summarized one particular theory that deals with growing old and dying. If you didn't catch my post, or the post were I referred you to the post, you can read a scientific paper on this very subject. It also discusses some previous hypotheses that were falsified. Because when I say formal, I mean mathematically based. Life and Death aren't mathematically describable entities. It is a fact not a law because you can't state it using formal language (ie maths)Actually, one could state it formally, but it would provide absolutely no utility. One couldn't use the equation (which would be the mathematical equivalent of the fact) to divine any information. It would be pointless, basically. There is no need for such a law. I know some laws (like the 0th law of thermodynamics) seem bloody obvious, but it is also required to correctly proceed with thermodynamics. The Law of Death would not be needed for anything. We can just refer to it as the fact of death. Your scenario was discussing organisms that lived longer and so their population sizes grow in response. I said that the population size might grow, but it would be limited to the resources available to the population. As such, life expectancy will likely deteriorate and genes which allow for longer life will cease to be positively selected for and would eventually cease their function and the population's maximum life span will be reduced.The effect? It would mean the population size will vary between being overpopulated to being underpopulated around an equilibrium point. Something we see to this day.Edited by Modulous, : No reason given.

Careful. If we don't get older until older - there wouldn't be more old people around. They were be more youthful people around. There'd be less old people around because more youthful people would die through predation/accident/murder/suicide etc.And of course, if you want to think of the scenario where senescence sets in at the same time, but takes longer to complete its course, we would have more old people around, but they would be more susceptible to predation/accident etc so they would still die.It is tempting to think of things from a human perspective, but we are a rare exception as far as animals dying of old age goes - I think it is only some of the larger/smarter organisms that get this accolade (turtles/elephants/primates). Mostly everyone else gets selected out of the gene pool much earlier But an organism doesn't care about its species. As long as it doesn't adopt a strategy that is directly harmful to its environment (such as killing off all mates), an organism that has a long life gene will live longer and will produce more children which can also live longer (even if - in the long term - that would be harmful to the species).
It would be advantageous for genes to create beings that lasted as long as is physically possible to spread as many copies of themselves as possible. Living longer means your alleles increase in frequency and as such it should be something that gets positively selected for.The puzzle then, is why don't we see such creatures?Edited by Modulous, : No reason given.

I wasn't refuting your position, I was advising caution with your terms, and then I expanded on your point. Indeed - the answer to the puzzle is two fold. If the population is at its maximum size then most animals are not dying of old age. Any genes that give a longer life span would not easily get selected.If the population is not at its maximum size though, and could support more members, where plenty of members are dying of old age, we still don't see this general trend and that is explained through the pleitropy paper referenced earlier. Genes that increase fitness early on are under more selective pressure than those that occur later on because there is more chance the organism will be eaten before getting the benefits.And that, as we say, basically covers the evolutionary reasons for old age.

It isn't about getting worn out since our components are continually replacing themselves. See also wiki:

quote:

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Historically, ageing was first likened to 'wear and tear': Our bodies get weak for the same reason that a knife gets dull or metal rusts. But this idea was discredited in the 19th century when the second law of thermodynamics was formalized. Entropy (disorder) must increase inevitably within a closed system, but living beings are not closed systems. In fact, it is a defining feature of life that we take in free energy from the environment and unload our entropy as waste. Living systems routinely repair themselves, and, in fact, can build themselves up from seed. There is no thermodynamic necessity for senescence. (Nevertheless, the idea of 'wearing out' has so much intuitive appeal that even experts will lapse into thinking that way at times.)

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Evolution of ageing - WikipediaThe current theory is Wilson's antagonistic pleitropy which I referred to in Message 6, though this is only partially validated at this time. One interesting thing is apoptosis or 'cell death' a process which cells kill themselves if they are diseased or faulty. This process goes a bit nutty in late life so that even healthy cells begin to kill themselves - this isn't 'wear and tear', it seems part of the natural development cycle of an organism for some reason.

Right - but the question is why do repairs and replacements stop happening? It's nothing to do with wear and tear because (as far as I can remember) nothing exists of my body from 15 years ago to have worn or torn. It's all gone, all of it. It wore out a long time ago. Nobody is disregarding environmental impacts...however the idea that cells are 'wearing out' over time like computer components isn't an accurate way to look at why we age.