How does evolutionary theory explain fixed biological lifespans? Clearly they are advantageous to most species as they tend to lead to genetic diversity. (That is, if a specific member of the species keeps reproducing forever and never dies, the population is more likely to be the same.) Yet, wouldn’t members of the species who live longer in general have more offspring? And consequently, wouldn’t those longest living offspring tend to mate more, ad infinitum until there was no limited lifespan? I would expect limits on some animals because maybe the metabolic rate would come into play, especially the more complex ones with fewer offspring, but shouldn’t some of the “lower” creatures evolved this simple advantage? (I mean, I’m glad we don’t have everlasting cockroaches, but why not?)
I expect you guys might tear this supposition to shreads quickly, but I’d be interested in hearing your responses.
(David - I figured since this may be a debatable evolutionary question, I put it here. Feel free to throw it to general questions if appropriate.)
There are a few ways to address this question. I think I will try 2:
Most populations exist in ecological niches where resources are finite. Increased lifespans lead to increasedpopulation which can lead to famine, increased violent competition, damage to a necessary ecological balance, etc. Therefore, it is by no means clear that increased lifespan would result in a long-term evolutionary advantage for a population. It may, in fact, be disadvantageous. The fact that lifespan is highly correlated with lineage, yet we have significant variations in lifespan among our population seems to indicate that increased lifespan by itself is not a strong enough reproductive advantage to be reinforced through selection.
How does evolution account for vulnerability to disease? Wouldn’t it be advantageous if a population were immune to all disease? It would increase survival rate and proide a reproductive advantage, so why hasn’t evolution made some species immune to disease? Obviously, this is meant as satire. Your question follows the line of a very common misconception about evolution. We can imagine any number of traits which might be advantageous to a population, but that has no bearing on how evolution works. In the first place, the fact that you can imagine a trait does not mean that it ever appeared in the population. Second, genetic traits do not exist in a vaccuum; advantageous trait A may have appeared in a population but been paired with disadvantageous traits B & C–thereby negating any reproductive advantage.
The best lack all conviction
The worst are full of passionate intensity.
*
Very few organisms have ever had the luxury of dying of old age. Predators, parasites, and disease take out most individuals in their prime. The successful taxons have had individuals who breed fast enough to keep ahead of the reaper; producing a number of offspring in a shorter time is more likely to be selected for than the potential to produce offspring over a longer period of time. All it would take is one flood or natural disaster and its bye-bye Methuselah, hello mayflies.
Longevity beyond that which is needed to bring the next generation into being is invisible to selection.
Longevity beyond that which is needed to bring the next generation into being is invisible to selection.
This is true, but some folks think that longevity in human females, and possibly other species, beyond breeding age is an advantage. The idea is that grandmothers, who can no longer breed, help ensure the survival of their grandchildren. Obviously this can only work with a social species, such as elephants or whales.
Excess longevity simply draws limited resources from the breeding stock’s use, so it should be selected against. If you’ve got food for 1000 animals and only 500 are at or below breeding age your reproductive rate is going to be reduced by half. Unless we’re talking about animals that practice birth control, which some do.
Also, as others have pointed out, animals in the wild seldom die of old age.
Many individual trees live a heck of a long time, producing offspring over the course of decades. Very few of those seeds ever attain reproductive age, but there is always a crop of saplings waiting their chance to reach the canopy.
So, it seems that there are some ways of life that do reward long reproductive lives.
There is also the fact that when you extend lifespans (in mammals anyway) you also extend (usually) gestation period and adolescence. Long lived mammals also generally produce a single offspring per pregnancy. Thus a female elephant will produce fewer offspring over a lifetime than a female rabbit will in one litter.
The fact that an organism’s DNA is subject to increasing damage with age should have some evolutionary effect on limiting lifespan. If not limiting lifespan, then, as the above post points out the value of grandmothers, there should be an evolutionary advantage to menopause and impotence in older organisms.
There are some pretty good answers here guys, but it leads to a couple follow up questions.
DrF mentioned trees as the possible fulfillment of this hypothesis. (Thanks, BTW, for a post that proves I’m not a total idiot—well maybe not PROOF, huh?) IIRC, there’s that particular bristlecone pine that’s estimated to be well over 3000 yrs old. Anybody know if it’s still producing seed? If its DNA hasn’t been damaged over the years, I’m wondering if mipsman’s “increasing damage with age” is limited to animals (not plants). Is aging more dependent on genetic coding telling me I’m old, or genetic damage forcing the issue?
Wouldn’t genetic “damage” from age be a benefit if mutations are created? Say an organism lives in a rapidly changing environment, and therefore is under pressure to evolve to adapt to that environment. If that organism produces several offspring with mutations, perhaps one of them will have a beneficial mutation that assists in its survival.
In a relatively stable environment, genetic stasis should be the norm. In a rapidly changing environment, those factors that increase the rate of mutation (including longer life span) would be favored.
quote: “Is aging more dependent on genetic coding telling me I’m old, or genetic damage forcing the issue?”
I like this idea, but does it have to be one or the other? If a dog’s muzzle turns grey as the dog ages is that because the of some aging gene or because some genetic expression is no longer producing the color to keep the muzzle the same color as the dog?
Oh, that seems worse. IF some genetic expression just got turned off it wouldn’t have to be the muzzle of the dog - could start at the tail, right?
Push that idea off the cliff>>>>>>
Quote:“Wouldn’t genetic “damage” from age be a benefit if mutations are created?” Aren’t most mutations destructive? Aren’t most genetic changes variations rather than mutations?
I’ve been told that we have a small variety of emzymes and proteins that correct genetic errors, help re-zip the genetic material back into the correct sequence. Gives me the feeling that mutations, although maybe not unavoidable are not appreciated either. Variations are the more acceptable method of change.
So these statements: “Say an organism lives in a rapidly changing environment, and therefore is under pressure to evolve to adapt to that environment. If that organism produces several offspring with mutations, perhaps one of them will have a beneficial mutation that assists in its survival.” would be fine if you changed mutations to variations?
The tree business was something I hadn’t thought about - trees must also die of old age? I know life spans for trees are given in some gardening books. But develop heart disease or diabetes, they don’t.
Oh, I’m gonna keep using these #%@&* codes 'til I get 'em right.
This is starting to get OT, but isn’t genetic mutation almost always a bad thing? I realize that currently there’s probably no way to determine the exact ratio (bad:good), as good mutations might be recessive, but if mutation is so good, we wouldn’t avoid radiation exposure. Wouldn’t it generally be in the species best interest not to have many mutated offspring, as it is unlikely they’d be able to survive?
Doesn’t it require a new mutation to create a new variation? Aren’t there just so many different buildings you can make from a given set of building blocks?
Asexual, budding creatures are, in a sense, immortal. When they reproduce the original organism doesn’t die, so every yeast cell in existance traces directly back to the original without a break.
They don’t have as much fun though.
I’ve read that carp don’t age (Heinlein maybe) but I’ve never found a reliable source for that.
And DrF, it’s an ice floe, an ice flow is a glacier. (Sorry, I couldn’t resist.)
I’ve thought about that a little, but I don’t know if you can consider the child the same organism as the adult. Identical twins are certainly not considered the same. Who’s to say the divided cell isn’t a different individual? In fact, who’s to say it doesn’t have some measure of intelligence that would distinguish it from other cells? Just because we have no way to measure it, doesn’t mean it’s not there.
“That Blobby is smart, but Slimey is such a jerk!”
When we harvest a crop, could it be like genocide, killing billions of entities? Just an off-the-wall thought…
Quote: Doesn’t it require a new mutation to create a new variation? Aren’t there just so
many different buildings you can make from a given set of building blocks?"
Yes, you’re right, darn it!
From a college biology text:
“Mutations are abrupt changes in the genotype-the ultimate source of the genetic variations studies by Mendel. Different mutations of a single gene increse the diversity of the alleles of that gene in a population. As a consequence, mutation provides the variability among organisms that is the raw material for evolution.”
