I was thinking that living creatures have actually no reason to die. If their organisms were made diferently, they could self-repair and live forever.
Then I realised that if there was a species that didn’t die, evolution would halt for them and eventually they would become vulnerable to some other competing species.
Is this true?
There are other ways to die.
[ul]
[li]Starvation[/li][li]Eaten by predator[/li][li]Severe physical injury[/li][li]Losing a fight for dominance[/li][/ul]
Death from old age is a luxury that most animals never experience.
Most organisms are food for some other organism, so if they do not die ‘naturally’ they are eaten. Many do some form of self-repair (get sick, get better; be wounded, wound heals). Also a species that does not die would take up space that should be used by their offspring.
If an organism is particulary long-living, one could assume that there would be, as you say, “self-repair” mechanisms. These mechanisms would at least consume some energy (or maybe could have other counter-productive side-effects). And they wouldn’t provide a significant advantage, since anyway, a lot of animals aren’t going to die from old age but rather from illness, during a drought, killed by a predator, etc…
If say, an animal is likely to live for 30 years, on average, before dying from a disease, being killed, starving, etc…Then a “self-repair” mechanism allowing it to live, say, for 100 years would give it an advantage only if it uses a very little amount of energy. Or else, it would be more likely that the animal would starve due to its increased energy needs rather than, out of luck, escape all other causes of death, actually live 100 years and have many offsprings.
This explanation makes a lot of sense to me. I did not make it up, but I know it’s disputed.
IIRC, there is an immortal sea urchin, lets see what google thinks: the results
And a site: like so
So there are “immortal” critters, but as I review I see that wasn’t your question, but this is my answer.
But you said it yourself, the critters that don’t die, and are unsuccessful in their environment, will be eaten, and the ones that are successful will continue to breed and introduce changes into the species (IANA anything warnings)
btw, immortal doesn’t mean doesn’t reproduce:
You assume the default condition is not dying. The default condition is actually dying right away. Succesful species manage to reach the dye afte reproducing stage. Further developments in longevity are side-effects of other adaptions for better reproduction.
Don’t think in terms of what would be good for the speices or the organism. It is often counter productive when thinking about why living things are the way they are.
Not really because that’s a serious misunderstanding of average life expecancy. Some marine turtles for example live on average less than 12 months before dying from predation and yet can live for a century or more. There are literally thousands of similar species.
Average life expectancy in most species is heavily skewed by juvenile mortality. Having a mechanism that allows them to live for centuries is totally unrelated to average lifespan.
Evolution does whatever it can get away with. It is only concerned with maintaining continuity. Evolution wouldn’t stop for a long-lived creature as long as it was still able to reproduce…if it’s offspring turned out to be better survivialists, then they would probably indirectly kill it by taking over its food supply.
Aging is simply a tug-of-war between the forces of entropy, and the forces that fight it. Most species put as little effort into this as they can and still be successfull. More than the minimal amount of effort doesn’t increase the odds of survival in the evolutionary survival of the fittest sense. It might even decrease the odds of reproducing since it is more energetically intensive.
Hijack: single celled things live forever if you consider one of the two they split into to be the same cell.
WAG: It could actually be a trap. It’s could be better for each individual to live and breed as long as possible, but this could lead to ‘immortals’ with almost no young no competed to death. And if some other threat came along, they wouldn’t be evolving to deal with it. And die out.
One thing to keep in mind is that nature doesn’t want things that are “better.” Nature wants things that fit in well with other things. If you get too good at surviving, you overrun your environment and starve to death. The things that survive in the long term are those that achieve a balance. The number that die off for whatever reason (eaten by other animals, or self destruct through aging or whatever) very closely balances with the number that are born. Large sharks, for example, are near the top of their food chain. They don’t get eaten much, so they don’t reproduce much. Bunnies, on the other hand, get gobbled up by everything in sight. Therefore they have to reproduce, well, like bunnies. Something that never dies would have to have a very low birth rate, or else it would have to get eaten a lot. Sea turtles are a good example of this. They have a very long life span, but as babies, they get eaten a lot. Very few make it to adulthood.
A low birth rate leaves you vulnerable to large scale disasters. If, say, a comet comes along and whaps into the area where these immortal beasties live, the animals in the surrounding area that have the highest birth rate will recover the quickest. Animals with a low birth rate could easily become extinct under such circumstances as animals with a higher birth rate beat them in the competition for suddently scarce resources. In this case, living forever could be very detrimental to the survival of the species.
Well, from an evolutionary persepctive, everything that you do after producing offspring is gravy. So, natural mortality doesn’t show up on the evolutionary radar, for the most part. All the things that cause natural death don’t have a significant impact on fecundity… and differential mortality after reproductive age cannot be subject to natural selection.
Unless, of course, your actions influence the fecundity or mortality of your offspring. If having hale and hearthy grandparents around improves the survival of grandchildren, then there could be some selective action on longevity and slower aging, though it would probably be fairly weak (i.e. only present in those situation in which the grandparents happened to live long enough for their good genes to show themselves and the grandchild that’s benefitting carries the same genes).
What would probably be a more common case outside of highly social organism, is that the grandparents (and possibly parents) are going to be competing with their own offspring for resources. In this case, mortality is an advantage. It has a double advantage for prey species, since large easy to catch meals are more appetizing that small easy to catch meals. Again, the selective pressure would probably be weak.
Far more likely is the already mentioned ecological concern. Species that live too long overpopulate and degrade their environment. The population dies off during the ecological collapse, and the cycle repeats until the genes present in the founder population(s) remaining after the last collapse have the right mix of genes to form a sustainable ecological mesh.
<nitpick alert>
“Nature” doesn’t exist and even if she (he) did, it wouldn’t care either way. Things just exist or they don’t, survive or they don’t.
<nitpick over>
Sure it can. Individuals in several species survive well past the first mating, and continue to mate (or try to) on a regular basis. Being longer-lived in such cases clearly helps in the “differential reproduction” department: if you live longer and therefore reproduce more often than an individual who lives just long enough to mate once, then dies, you have a definite selective advantage (assuming whatever granted your increased longevity was inheritable).
OK. Make that “average life expectancy of individuals reaching mating age”.
Yes, that’s what the rest of my post was about. 
I think you’ve misunderstood what “reproductive age” means. For humans it’s roughly from 15 to 50 years. “After reproductive age” refers to the time when the organism is no longer likely to produce offspring… not from the moment they become fertile, or the moment they reproduce.
Fair enough, but that’s more a case of having a longer reproductive age v. having a longer life. Also, living longer != more offspring. You can reap “differential fecundity” benefits by splurging all your biological resources: you only have one reproductive cycle (which kills you) but you produce hundreds of offspring. BUT, within a species it’s only going to be splurgers competing with other splurgers or serial breeders vs. other serial breeders- you probably aren’t going to find a single species with both plans in being practiced.
Natural selection could really only extend reproductive age about as far average life expectancy… If you only live 5 years, it doesn’t matter if you could theoretically have kept pumping out the pups up to 7 years, or if you could have done it to 70.
OK, but that’s still completely worng for the reaosn;s Darwin’s Finch has outlined. Average age isn’t of any real significance here.
I do not believe so. See here, for example:
If “reproductive age” were just a range, those individuals aged at 30 years would still have been included in with those of “reproductive age”. See also here, under the definition of “fitness” (one does not “reach” an age range except by first reaching the minimum age). “After reproductive age” would therefore be any time after that which the individual was first able to reproduce.
There is no “official” definition of reproductive age that I have been able to find, and it appears different authors use the phrase to represent different things (and the only cases I have seen where it is used as an age range are in reference to humans).
Darwin’s finch I’m going to have to agree with 1010011010 on this one. He did say “after reproductive age”, not “after reaching reproductive age”. While you’re right that an individual reaches reproductive age by reaching a minimum figure they aren’t after reproductive age until they have reached a maximum figure.
The example you gave of dividing people into reproductive age and those who are 15 years older seems to me to simply be implying that people 30+ aren’t reproducing because they are too weak or simply post-menopausal. This is further supported by the rets of the article:
The fact that the definition of fitness supplied speaks of reaching reproductive age doesn’t really say much either way, since we all agree that reproductive age has to be reached. The dispute is whether it continues from the age it is reached until the organisms can no longer reproduce as 1010011010 says, or whether it is only that one age as you suggest.
Consider these examples which make the term much clearer:
www.sti.ch/pdfs/zinsstagpart6.pdf
www.uco.es/organiza/servicios/ publica/az/articulos/2002/196/pdf/goyache.pdf
http://www.fao.org/wairdocs/ilri/x5473b/x5473b23.htm
If “reproductive age” were not a range then it would be meaninglesss to refer to a beginning of reproductive age or an annual production of offspring from reproductive aged individuals.
But it’s a pedantic point anyway, so long as we all can agree on a definition it hardly matters.
Blake, I seem to recall seeing you in another biology thread… 
I’m willing to accept there are multiple possible meanings of “reproductive age”, and it’s not important whether there is a single official correct definition and/or whether we’re using that one. If we can all agree which definition to use within this conversation, or specify what meaning we’re using, it shouldn’t be an issue.
Darwin’s Finch, with the understanding that I meant the range of years were an organism is fertile, do you have any other comments on my post(s)?