Death and reproduction

Great Debates
1

I have been reading a review of a book: 13 Things That Don’t Make Sense: The Most Intriguing Scientific Mysteries Of Our Time by Michael Brooks. It sounds interesting, and perhaps the answers to the following question will be revealed when I have the book; but in the meantime, can anyone explain this:

The review says: ‘Brooks also tries to solve the puzzle of death. Why do we die? The answer, possibly, has to do with something else we don’t really understand - sex. Still, there might be interesting connections between sex and death. Sexual reproduction involves the swapping of genes, which enable species to adapt to their environment. The faster the turnover of generations, the quicker the adaption - so shorter lifespans can be good for a species. Hence death.’

My question here is that surely it isn’t the lifespan of an individual that affects the the turnover of generations, but how long it is before they mate. The sooner they mate, the quicker the generational turnaround, surely. Long lived people may present other issues, but they don’t stand in the way of reproduction. Or is our desire to reproduce influenced by how long we live?

2

Genes aren’t selected for the “good of the species”. They are selected by their environment for their own ability to reproduce and persist. In sexual organisms, however, any effect a gene may have must be expressed before or during the organism’s child-bearing years for selection to occur. Genes that, say, cause cancer in 80 year old humans aren’t going to be selected against, because they had no effect on the person’s ability to reproduce. Therefore, genes that kill people tend to be expressed in our later years and there is nothing we can do to change that. As we get older, more and more deleterious genes are turned on, until eventually we die.

It is for the good of the species that our elderly eventually die, but that isn’t why it happens. If a gene could find a way to live longer and reproduce more by delaying or eliminating death, it doesn’t matter how detrimental it would be to the rest of species, that gene would succeed and spread.

Furthermore, in most species, including humans until recently, death by old age is almost completely non-existent. So it is kind of ridiculous to say that it is selected for on purpose for the good of a species.

3

Not necessarily. Since in every environment there is a limited amount of food, long lived individuals (defined as those past the age of reproduction) might be using up resources without contributing to the reproductive success of those younger.

4

That isn’t right. For example, helpful grandparents can improve the odds for the genes they have already long ago passed down through two generations. In fact elders can in many ways help the success of the population into which their genes are spreading.

5

I see a logical fallacy here:

Why should an organism ever be past the age of reproduction? Why shouldn’t it remain perpetually young and keep churning out offspring until it gets eaten or dies of disease? On a naive basis you might suppose such an organism would be more successful in leaving descendents. If anything, it seems to be the other way around: the age of death is set by the age of reproduction.

Sorry but I’m going to have to directly contradict you. Many organisms spawn once and then immediately seem to fall apart- salmon are the premiere example but there are many others. And for others it’s simply not true that aging is irrelevent. The vast majority of organisms get eaten, starve to death, or die of disease before ever reproducing. The ones that DO make it to maturity and successfully reproduce then usually die of age-related causes, such as diminished fitness or weakened immune systems. It sure looks to me like organisms are programmed to die, and I have no idea why that should be.

6

That’s because they’ve on average already successfully reproduced. There is little reason (from the gene’s perspective) to keep them any longer. and it doesn’t matter how they die, just that they get out of the way eventually

Related: say some animal would all have the potential to live 1000 years on average, and be able to reproduce up to, say, 600 years, most of them would still die out much earlier than that, just because they’d run out of stuff to eat. You have to balance the reproduction rate with the death rate - by natural or unnatural means.

7

This understanding of the aging process is almost entirely incorrect. There may be a handful of genes that work as you describe, but for the most part the aging process works quite differently. Aging does occur because we have certain genes that are dormant throughout are youth and then become active in old age. Rather, it occurs because, as we live, damage gets inflicted on our bodies in various ways. Some of the damage can be healed, but much of it cannot. The longer we live, the more damage we accumulate, until eventually it kills us.

Cancer is one good example. Most cancers don’t occur because of genes turning on, though there may be a few that work that way. Rather, they occur because genes get turned off. Breakdown of DNA eventually damages the genes that regulate cellular reproduction. Once enough of those regulators cease to operate, it becomes possible for a cancer to grow.

Other examples of the accumulating damage in old age include thickening of the artery walls, loss of bone tissue leading to osteoporosis, and degeneration is the muscles that control the eyeball.

8

Perhaps the reason is genetic diversity. Reproduction by those with new genes is more advantageous than for those who have already reproduced.

Single cell creatures can’t really be said to age beyond the ability to reproduce.

Your point about grandparents is good, but really only applies for humans - and it might explain why humans live relatively long after prime reproduction age.
Human eggs and sperm age, and become less effective. (Eggs age, and it appears the creation of new sperm becomes more error prone with age.) Does anyone know if this is also true for other animals? That might be the thing that makes old animals no longer useful.

9

Except, perhaps, to reproduce some more??

Why can’t they reproduce up to 1000 years? You can’t say “because they get too old”, that’s circular logic. Why should they age?

10

Aging is a consequence of dealing with the affronts of the world - radiation, errors accumulated in copying cells, etc. The question is when is it worth investing metabolic resources in dealing with it and to thereby have a longer lifespan?

In the case of each organism there are different and conflicting forces at work and coming to some balance:

Does an aged individual have the ability to continue to attract potential mates and therefore to continue to spread its genes around?

Does the presence of the aged individual increase decrease or have no effect on the potential future survival and reproductive success of the next generation(s) and thus the gene’s long term “success”?

How much impact does the metabolic cost of undoing the damage of aging have on the individual ability to survive and not die from other causes?

With those issues in mind let us examine the cases of some of the longer lived organisms.

Certain trees can live to thousands of years old and can reproduce into old age. One such tree is the bristlecone pine. It has little competition and little to live off of. From the gene’s POV (so to speak) the answers are: it needs no mate; the individual’s persistence does not impede any seed that happens to land in another random hospitable enough spot many meters away; and it has very specific low metabolic solutions to deal with the effects of aging.

Some antarctic sponges live hundreds of years and seem to be in comparable circumstance.

In both these cases it seems that the toughest point to survive is youth and once in adulthood the genes’ interests are served by producing young consistently over many years a few of which may survive with the extant adult neither helping or hurting that process.

Among mammals longer lifespan belongs to whales, elephants, and us. Each of which invest much energy in bringing our young up in groups that benefit from some sort of “culture” in a fairly prolonged childhood. The ability to attract mates may diminish but the net benefit of to the next generation may providing for and protecting the young until they get to the point of reproduction age themselves was selected for in an offsetting manner. On an evolutionary scale the balance of metabolic costs dealing with the consequences of time made most individual humans die soon after the age their children made it to reproductive age themselves.

Short lifespans OTOH seem to correlate with profligate breeding and little investment in the young. In fact often direct competition with them and the gene’s benefit served at least as well by having the young win that competition. Many insects come to mind. Adulthood brings no survival advantage and predators can (and usually will) get you quick. Get to reproductive age fast and invest all metabolic energies into getting the genes out fast because you won’t get a chance to live long anyway.

11

On reread it is of some potential note that this happens within our own species as well, in a behaviorally flexible manner. The consistent pattern is that humans tend to have more children, very soon after reproductive maturity, and invest less in each child, in circumstances in which there is a high death rate whether it be from war, poverty, famine, or crime, and that stable economic circumstances and longer average life span are correlated with decreasing fertility rates and delayed onset of reproduction. (See this thread.) Interesting.

12

I don’t see any evidence that this is true, or rather its is true, but it’s entriely an accidental correlation. Firstly small size classes are swamped by invertebrates at the very smallest scales and then by reptiles, then by mammals. So you are drawing a correlation between clades, which is invalid in its own right. Then short lifespan correlates well to size because of surface:volume issues, and small size correlates well to an r-strategy because smaller animals have more predators. But lifespan and reproductive strategy are both due to size. They are not directly relate dot one another.

You can see this quite simply by controlling for size and clade. If you look at the insects, the longest lived individuals are termites that produce hundreds or thousands of reproductive young every year for 50 years or more. In contrast ephemeral insects such as solitary bees or moths live less than twelve months and produce only a dozen or so offspring. So no direct correlation between lifespan and reproductive capacity.

Move up to the tiny mammals and you’ll see the same thing. The shortest lifespans, less than a year, are found amongst the smaller dasyurids, closely followed by some of the shrews. These dasyurids produce only two to four young each, the shrews four to six. In contrast the mice are the same size, and they live for 5 years or so and produce 12-30 young annually. So no correlation whatsoever between lifespan and breeding strategy.

Similarly for large mammals, we have anything from annual litters of six for pigs (lifespan ~12 years) to single annual offspring for sheep (lifespan ~10 years).
And we could show the same lack of correlation within size classes and clades for the fish, reptiles and so forth. Basically I’m just not seeing any direct correlation between lifespan and reproductive strategy. Any apparent correlation seems to stem from an invalid lumping of clades and sizes together. I would have to see some evidence to believe that the correlation exists within clades and within size classes.

The trouble with this is that when we look at those insects where adulthood does bring a survival advantage and where predation does decline with age, such as the termites or many of the orthoptera, they don’t produce any fewer offspring than thsoe insects where that is not the case.

That is true enough as far as it goes, but it is equally true of insects like termites or long-lived orthopterans. Producing lots of offspring is always a good evolutionary strategy regardless of individual lifespan. Similarly investing a lot of time and effort into caring for very offspring is also always a good evolutionary strategy. Which is why ephemeral insects such as solitary bees and hornets spend vast amounts of energy constructing chambers for their dozen or so offpsring and stocking it with food, even though the adult lifespan is as short as that of any insect.

13

How many children on average do those long-lived sea turtles have during an average breeding cycle and how many breeding cycles per century can they expect?

Seems relevant to the discussion.

14

Blake, I would think that colony species are a special case and it is unclear if the reproductive unit in that case should be considered the individual, most of whom never successfully reproduce, or the colony itself (for which the successful kings and queens function as the equivalent of gametes.) Your point about the solitary bees however is a valid one. My correlation regarding short lifespan and profligate breeding may be incorrect. OTOH I think the analysis regarding longer lifespans is a valid one and since posting earlier I found cite of note.

I disagree strongly with your last bit. No it is not always a good strategy to produce many offspring. No it is not always a good strategy to invest much into offspring. In fact the two tactics are at odds with each other in many cases … even in the case of the solitary bees - who invest by providing for each young and therefore have fewer of them.

15

I can give you a spiritual answer. Death is falling to a lower level, underground/Sheol/the abode of the dead. Birth is the coming back up from underground. Through one man (Adam) all die, Eve means mother of the living. Man is required to give up his life by God, women are required to bear children by God.

I believe the common saying ‘go to the light’ (after death) may mean to be placed into another body, not going to God.

16

I somehow thought you could. :slight_smile:

You don’t really address the issues in the OP, and I can’t see any correlations between what you and I have posted. Although I know your mindset doesn’t allow you to entertain other viewpoints, I was inviting a discussion on the generational influences on evolution. Although I am sure it is not your intention, your post here invites a derailing of this thread, so I would politely request that you reserve your input for subjects that are more directly relevant to your experience.

For example, this

has absolutely nothing to do with my OP. And that is a fact, not an opinion.

17

The goal of reproduction is not to swamp the globe with unviable young. From a gene’s point of view, once its future is secured, it doesn’t need the parents. For an animal in a more or less stable environment, you only need a handful of kids that make it to adulthood. But most animals reproduce as much as they can anyway.

If that species suddenly would be able to reproduce a lot more (again, in an otherwise stable environment), that would mean they’d be competing for resources with their own descendents. Probably, practically the whole population would be at the edge of starvation within a few generations because there would be nothing left to eat. This doesn’t seem like a very successful evolutionary strategy to me.

Fine by me; let them reproduce up to 1000 years. Won’t change the argument.

I don’t think there’s even a problem with living a 1000 years, as long as each animal still produces only as many viable young as before. You only need to replace the dead.

18

I was answering the Q that you posed in the 2nd paragraph, the link between death and sex/reproduction. OK it’s Brooks’ question, not your that I was addressing.

19

No: you were stating facts according to your beliefs. You weren’t debating the statement that

You haven’t mentioned speed or adaption, or whether or not shorter lifespans are good for a species.

Like I’ve said, either convince me I’m wrong, or don’t post non-sequitors.

20

I’m trying to dredge up some memory from my academic youth, since I do not have time to search for a cite right now.

As memory goes, we don’t die from accumulated insults, but by pre-programing.

Single celled organisms are effectively immortal; they reproduce by cell division, and can continue to divide (given proper conditions) forever. But multicellular organisms have cells with an internal counter. Each cell can only undergo a finite, and rather specific, number of cell divisions. As I recall, for humans this is apparently somewhere around 100. After that, there are no new replacements for older cells that have become dysfunctional.

In a zygote, all cells are stem cells, and like single celled organisms, effectively immortal. But once they differentiate, they are locked into a certain number of further cell divisions, after which they will be unable to divide again. Senescence of the organism occurs as the cells in various tissues and organs lose the ability to replace themselves with newer, fresher models. Again as I recall, this “clock” is somehow mediated by histidine and non-histidine proteins.

A cancer is thus a de-differentiated cell. It has lost its original purpose (skin cell, liver cell, whatever) and also lost the inhibition against unlimited cell divisions. It divides without any control on either its “purpose” (as part of a tissue) or its numbers.

This explains the fact that humans (modern, not biblical) run out of clock somewhere just after a century, regardless of the conditions under which they live. No one, no matter how careful they are about their diet, no matter whether they reside in a bunker, can exceed this built in programmed death. Oh, an individual might live longer, but not beyond that seeming maximum near a century and a quarter. No one, no where, ever exceeds that.

As I also seem to recall, strains of mice can be bred for longevity. A strain whose members all die off around 2.5 years and no members of which ever exceed 3 years can, by selective breeding, produce a new strain that can live even longer. The whole bell curve, as well as the end point, can be advanced.

Supposedly, these histidine and non-histidine proteins and the control they exert over cell replacement could control immortality (or at least vastly enhanced life expectancy).

Perhaps this explanation has ben discredited since I last heard about it-- but damn, no I’m gonna have to go look it up! (Unless someone out there in SDMB-land beats me to it. Hint.)