SULPHUR

I believe most of the points you are trying to make are simplistic and have been covered above.

Lobstrosity

Because without an adequate death rate populations are dramatically unstable and an unstable population is extraordinarily succeptable to extinction. Organisms that did not evolve this way would have been removed from the gene pool long ago by selective pressures generated by the population itself. Adhering strictly to the stance that natural selection deals only with the individual is grossly naive and is only a first-order approximation of the process of evolution. Individuals are coupled to each other via their population. This coupling would be a weaker, second-order effect, but it is there nonetheless. Think about it this way:

Long ago you have two isolated populations of nearly-identical creatures. In one population has creatures that lose fitness with age whereas the other has creatures whose fitness is time-independent. This latter population, without predation, goes through large-amplitude boom-bust cycles whereas the former has such cycles greatly smoothed by adding a death rate (essentially aging acts as an integrator). Now, subject both populations to a dramatic climactic shift, such as an ice age or meteor impact, and ask yourself to venture a guess as to what will happen. If this climactic shift catches the "immortal" population during a bust, the likelihood of extinction is significantly higher than it is for the stable population (since the "immortal" population has far less individuals and hence far less genetic diversity). The climactic shift is more likely to erase all genetic material from the non-aging population than it is to erase the genetic material from the aging population.

In short, individual selection might push organisms towards immortality, but such moves probably drive those organisms' populations to extinction through destablization of that population; hence we do not see many such organisms in existence today. What we see alive today are those populations that can exist in a stable equilibrium, and a death rate is required for this. How could an individual evolve to age? Well, it's not that hard to come up with a scenario for that. Aging doesn't impact a creature's survivability so long as a creature's genetic lifespan is on the same order as its natural lifespan (as dictated by disease, injury, predation, etc). "Neutral" genetic mutations leading to aging could very easily enter a population and survive in non-negligible numbers. Members of this population then scatter to found new, isolated populations. Founders who age produce populations that age. Founders who don't age produce populations that don't age. Fast forward a few million years and you probably have more aging populations left than non-aging populations due to asymmetric extinction rates between the two different types of populations. Now reproduce the process a few million times, throwing in a few asteriod impacts and ice ages for good measure, and I could easily see us in a world of creatures that age.

There are many reasons to hypothesize that aging is due to gentics. First and foremost is the fact that different animals age at vastly different rates. There seems to be no set "break-down" rate for living cells. Each species has it's own "break-down" rate. Also, one must note that at least one human genetic disorder can lead to highly-accelerated aging: Werner Syndrome.
Aging clearly can be genetically controlled. Is it? Who knows. All I know is that you shouldn't dismiss the possiblity outright.

Edit: Here's an interesting article I just found. It suggests that there aren't specific genes for aging but rather specific genes for survival that happen to be imperfect. In my mind, the question left to be addressed is whether this imperfection is (a) there because this is just the best we could do given perhaps the costs of trying to do better, or (b) there by "design" as something that in some very roundabout way was selected for (see my last paragraph).

I said that death allows for reproduction, with the implied meaning that death allows for reproduction without massively-unstable boom-bust cycles.

I understand that at its core evolution is driven by selective pressures on the individual, but this does not mean that group interactions can be wholly ignored. For example, altruistic social behaviors do evolve. I personally feel that you simply are too willing to neglect higher-order effects of complex population dynamics that are in truth not negligible.


In short, evolution does not always drive individuals towards increasing fitness. Because of the influence of sexual selection, it seems that sometimes it can be destablizing with detrimental effects on individuals and hence on populations as a whole. For example, one story I heard a few years back in a biology class was of a species of elk (or some like-antlered creature) that progressively evolved to have larger and larger antlers. It seems that the females of the species were impressed with size...bigger was better. The antlers grew quite large, taking up valuable nutrients and making survival difficult. Extinction followed.
Edit: google has turned up this. I believe my professor was talking about the Irish Elk. It seems it's by no means conclusive that extinction was due to antler size, but it does not seem unreasonable that the massive antlers were a contributing factor. It's probably hard to deal with major climactic upheaval with these huge 80-lb prongs on your head, stealing vital nutrients and giving your head loads of inertia.

This is my philosopy: evolution is driven by natural selection on the individual. This natural selection is a combination of environmental and intrapopulation pressures. Sometimes these pressures drive individuals into states that destablize the population and lead to extinction. My view is that evolution isn't always a stablizing agent. When it becomes destablizing, extinction comes in, cleans up the mess, and that course of evolution is forgotten. What we're left with is only that evolution which led to stable populations. I guess you could call it natural selection of evolution on the population level.

Doubting Didymus

You present an interesting take on species selection. species that just happen, by chance, to be less susceptible to extinction are the ones that we see today. This is not because of any hypothetical evolution towards extinction immunity, but a simple result of history. The immortal species would be more likely to go extinct, the species with individuals that failed to evolve the adaptive immortality trait just happened to be more likely to survive the unplanned extinction event.

So, if this is the case, the individual in the surviving species that does evolve long lifespans is still at a selective advantage over his siblings, correct?

Lobstrosity

Correct. Living longer and reproducing more would convey an advantage to the individual.

I guess the main idea I'm putting forth for scrutiny is the idea that populations can be considered as entities in themselves. Just like organisms reproduce and die, populations as a whole will, over much longer timescales, reproduce and die. Populations evolve within themselves, but they can also fracture into multiple lines, each a separate, distinct population. On top of this, some populations suffer extinction, producing an evolutionary dead-end. I would propose the populations with genetic attributes most resilient to extinction are the ones that "reproduced" more and are the ones that much of the life on Earth today evolved from. I'm actually working through this hypothesis as I type it, so I'm sure there are all sorts of logical holes here, but I don't think I'm entirely off base. I don't think dynamics at the population level can be wholly ignored, and I think extinction has played a tremendous factor in course life has taken throughout Earth's history. Just like bad mutations cause the individual to die and hence are not spread througout the population (i.e. are not selected for), "bad" populations go extinct and their genes are not propagated forth--no future species will call this population their ancestor and thus the particular genetic developments that made that population unique are lost. Thus the species we see today have been selected in the sense that they came from that which was able to avoid extinction. I don't know...I'm probably talking out of my ass here.

Doubting Didymus

Hmm. I'm not certain how accurate it is to speak of species reproducing. For that to be meaningful, there would have to be features of the species, that is, not just of the individuals within it, that can be passed on to daughter species with fidelity.

However, I don't think thats what you're really saying. You're talking about the shadow of history on the modern biodiversity, if I'm not mistaken. I don't think species need to be framed as entities in order for that to be a worthwhile hypothesis.

Of course it has, and I can definitely see your point about population flux in an undying species. Earlier, I imagined you were talking about some kind of adaptation that confers a resistance to extinction, which I would reject. I now see that is not your point at all. I would phrase your argument, as I understand it, thus:

"Owing to the theoretical tendancy of undying species to become extinct more often than species that age, the history of biodiversity is biased in favour of species that fail to come by life extending adaptations."

Having phrased it like this, I notice that while the hypothesis may explain why the current batch of species we see is not undying, it does not, on its own, explain why a species would develop ageing in the first place. That is to say, the hypothesis does not supprt a notion that a species would evolve ageing to exploit this extinction resistance advantage, just that species that happen to age, or that happen to fail to evolve extreme longevity for some other reason, are more likely to be seen in the modern age than the other.

For myself, I reckon that no species ever evolved undying abilities as adaptations in the first place. The traits that would give rise to it would not, in my opinion, have very much opportunity to get a test run, in a world where the vast majority of creatures are killed, rather than die of age.

NZAmoeba

Evolution is a pet topic of mine, so i'm gonna quickly dive in here and give a nutshell responce, as this is getting quite verbose...

If you get to breed at all before you die, you're a success, that is how evolution stands, anything that happens to you afterwards is irrelevant.

Death has nothing to do with evolution, i remember seeing a british documentary on the topic of ageing that summed it up nicely. As you know, the cells in your body are constantly making copies of themselves, and eventually self destructing.

When you make a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy of a copy.... you will eventually get a mistake. Eventually the number of these mistakes becomes too much for the body to handle, something critical will give way, and the body will die. This is why death happens.

Evolution has done nothing about this, because this process takes a while, a lot longer than it takes to reach maturity for breeding, and because evolution doesn't care what happens after breeding, it's ignored.

and thats my summary for anyone still going 'hunh?'

Now i'm going to bring up that old horse topic mentioned before. Horses use'd to be small dog sized, and their main predator used to be a giant bird (like an emu). They also lived in dence forests, so for survival, the horses needed to be small enough to run through the deep undergrowth unhinderd.

The giant bird died out, and the forests dissapeared, this left large open plains, and wolf like creatures to become their main predator. They didn't have any cover anymore, their only hope was to run, fast. Now, to run fast requires a very large step, so the larger horses lived, and we eventually got the refined sprinting machines that we have now.

Look at shetland ponies, they're like mini-modern horses, the shetlands obviously were predator free, so the horses didn't need to run from anything, so they stayed small, as growing big takes a lot of resourses, and the shetlands arn't big at all. (ps. they could have just been bred that way by someone, i'm assumeing here, but why would someone want a tiny horse?)

I'm done now.

Deke

Malookiemaloo restated the original question: Still don't think science explains WHY death exists though.

Science doesn't explain WHY life exists either. Scientific inquiry can tell us the mechanisms of life and death, but I suggest you're making a very common mistake of expecting that scientific inquiry should supply meaning as well as description.

Evolution is a very good model for explaining the speciation of life on earth (or HOW things got the way they are), but the theory makes no claim to having an answer to WHY life is here. Also, I would offer that the idea that evolution produces "better" species is an inaccurate way of thinking about the process of adaptation. "Better" simply means able to exist in the current conditions. A small horse was better suited to its environment, and as horses became larger over time, they were better suited to their environment. Human beings cannot live in the dark deep waters inhabited by angler fish - does that mean they're better than humans? Of course not, no more so than the idea that humans are better than angler fish because we can live on land.

I would also offer that while individual life forms age and ultimately die, the material of which they are comprised does not vanish from existence.

MHO,

Deke

Peez

Why? Do you have any kind of reference for this?If I understand you correctly, I agree here. However, how would this influence the evolution of the population?Please explain. Why would the probability of extinction of the population influence natural selection within the population?As I have a Ph.D. in evolutionary biology, I suspect that I am not at all naive about evolutionary processes. Rather than making empty claims, please address the points raised.??????Presumably we are assuming that all other factors are equal. First, you have not established that a population of immortal individuals is any less stable than a population of mortal individuals. However, for the sake of argument I will accept this unsupported hypothesis. Second, if the population is unstable it might just find itself larger than the other population when the crunch came, but I will ignore that as well. Certainly if the population is smaller it is generally more prone to extinction, just because there are fewer individuals available to survive and reproduce. Whether or not genetic diversity in the population would influence their chances of survival depends on a number of factors, such as whether or not the availability of genetic variance is a limiting factor to evolution in these cases. Interestingly, you have missed a much better reason that the immortal population would evolve more slowly: it is hard to change allele (gene) frequencies when individuals live a very long time (since the alleles in those individuals are obviously not changing).

Unfortunately, this entirely misses the point. Going back to your example, let us imagine that an individual from the immortal population makes it to the mortal population, and visa versa. Until the crunch comes, the mortal individual will reproduce at a lower rate than all the immortals in that population, so natural selection is favouring immortality and the mortal genes will eventually disappear. At the same time, the immortal individual will have a higher reproductive rate than all the mortal individuals in that population, so natural selection would favour the immortal genes and they would eventually take over.You have yet to establish that immortal populations would be unstable, but again even if this was true immortality could still evolve in populations if there was no cost.Please explain why a death rate is required for population stability.You seem to be assuming that all individuals have exactly the same "natural lifespan." Otherwise, this statement doesn't make sense.They would only be neutral if no individuals lived long enough to age.If all your assumptions were correct (I don't believe that they are), then this might be a logical inference. What you are presenting is a group-selection model. As I discussed earlier, group selection does seem to work under very restrictive conditions, but those conditions do not seem to be met here.Before you go off on this argument, you first have to establish that the assumptions that you are making are reasonable.I am not arguing that genetics has nothing to do with aging. I am certain that it is very important.This is actually very poor evidence, as different animals also experience vastly different environments.This may be true.Not entirely unreasonable.To be precise, it leads to signs and symptoms of highly accelerated aging.I did no such thing. I am certain that there is a genetic componant to aging.Of course if the latter it would not be an "imperfection." If reproduction can occur without death, in what way does death allow for reproduction? You stated:All you need for evolution is for individuals to reproduce, which they can do fine without death.I am not ignoring them, but you do seem to be ignoring selection on individuals.These can evolve quite nicely without group selection (Edit: depending on how one defines "altruistic").I have just explained that selection on individuals is stronger than selection on groups except under very restrictive conditions. I have also explained how lifespan could evolve given trade-offs, but you have not explained precisely how immortality could even be selected against at the group level.Would you care to explain this?Sexual selection is simply a subset of natural selection. Sexual selection has no more "detrimental effect on individuals" than other forms of natural selection have.The story of the irish elk has often been exaggerated, but the point is that natural selection (specifically sexual selection) appears to have been more powerful than group selection in that example.Note that millions of species have gone extinct, and most of them did not have big antlers (or anything similar).For the evolution of adaptations, I would say that this is generally true.To clarify: this natural selection is a result of interactions with other members of the same species, members of other species, and the non-living environment.If you mean "sometimes the population goes extinct because they cannot survive in the environment, living and/or non-living," then I agree.I certainly agree.Just because it is not stabilizing does not make it destablizing, but I do agree that it can be destablizing.O.K.That would depend on whether or not selection within the populations favours stabiltiy, how often populations go extinct, and how often new populations are started, to name a few factors.Yup. Look up "group selection" or an ecologist named Wynne-Edwards.

Peez

Peez

You are not off base at all. This is very insightful, but Wynne-Edwards beat you to it by about 40 years. However, this group selection (as far as we can determine) is much less powerful than individual selection in nearly all situations. To put it simply, the "lifespan" of a population is almost always much longer than the lifespan of an individual in that population, the "reproductive rate" of a population is almost always much slower than the reproductive rate of an individual in that population, and the "mortality rate" of a population is almost always lower than the mortality rate of an individual. As a result, a population of individuals can evolve far faster in response to individual selection than is possible for a metapopulation of populations responding to group selection. If immortality was an unqualified advantage for individuals, we would expect it to evolve in populations faster than populations reproduce or go extinct.

Peez

gcameron

Yeah, I've thought about that. So what happens with a single celled organism that reproduces asexually? It splits, and then there are effectively two "newborns"? Does "aging" mean anything in the context of a bacterium, or is this a process only seen in multicellular creatures? How about wear-and-tear, scarring, the general flotsam that affects a body as it wends its way through the weary path of life? Does a bacterium get affected by any of this, and if so, does any of it pass through to the next generation? Or is the one "old" parent erased and replaced with two "young" babies? Does "mommy" bacterium effectively "die" and become two "baby bacteria"? Or is the distinction between a "young" and "old" bacterium meaningless anyway...?

Of course each of us is the product of an unbroken string of successful cell reproductions going back to the dawn of time. We are a phase in a larger immortality, though "we" (meaning the individual body-vessel) can die. And then there is the paradox that, although each of us must be the scion of an unbroken string going back millions of years, that string can be broken the moment we step into the path of oncoming traffic...

Heady stuff...