Aging is the Cost of Species Adaption and Survival
Permalink | View Comments (10) | Post Comment | | Posted by Reason

A small number of species have exceedingly long life spans and show few signs of degenerative aging, so clearly biology is up to the task of continual repair and vigor. Yet the vast majority of species consist of individuals who age, and who will die because of aging should they survive the many other causes of mortality in the wild. Why do we age to death? The present consensus is that the prevalence of aging is the result of an evolutionary arms race to the bottom. Species that age better adapt to changing conditions and thus will take over most evolutionary niches. In effect we age because the world changes. Some thoughts here from a researcher in the field:

In the long run, the ability of a species to evolve is more important than anything else in determining its competitive success. This is true almost by definition: given enough time, the ability to adapt and improve will overtake any initial disadvantage. But evolutionary theory these last 50 years has been quite skeptical of "in the long run". If it is driven to extinction because of a competitive disadvantage in the short run, then what matters if it has the potential to improve, eventually?

This has everything to do with aging. A population with aging has more diversity and a faster turnover compared to a similar population in which death is only due to famine, predators, disease, etc. So - in theory - a population with aging evolves more rapidly than a population that doesn't age. But "the long run" can be thousands of lifetimes, and in the meantime those individuals that die early (of aging) are at a competitive disadvantage compared to those who continue to live, and have that much more time in which to produce offspring.

Can an aging population resist invasion (by longer-lived competitors) and cohere long enough that its superior rate of adaptation turns into a decisive advantage? This is the question that has been at the center of my research the last dozen years. On the one hand, there is abundant evidence that aging is no accident, that it has evolved via natural selection that explicitly favors aging. On the other hand, the theoretical argument casts doubt on the scenario where aging is selected on this basis.

The best resolution I have been able to find for this paradox is that aging has been able to evolve on this basis, and it is because the short-term advantage of unrestrained reproduction has been held in check by a different, faster-acting evolutionary principle than evolvability. Unrestrained reproduction leads to population overshoot, population crash, and extinction. This is a powerful, fast-acting evolutionary force, and populations have had to adapt by tempering individual competitiveness. This has created an environment in which the long-term advantage of aging is relevant, and aging as a population-level adaptation can thrive on this basis.

Link: http://joshmitteldorf.scienceblog.com/2014/08/06/adapt-or-die-adapt-faster-and-beat-the-competition/

Comments

Isn't this contradictory to your previous views on the topic?

"A small number of species have exceedingly long life spans and show few signs of degenerative aging, so clearly biology is up to the task of continual repair and vigor." Those species are generally much simpler than humans, so they don't provide evidence that biology is "up to the task" of "continual repair" for humans.

"In the long run, the ability of a species to evolve is more important than anything else in determining its competitive success." Tell it to the coelacanth. Moreover, if this were true then we would see that speciose clades thrive while relict clades are few and prone to extinction. That's contrary to the observed topology of the tree of life. Paleontology is replete with examples of big bushy branches that were lopped off the tree wholesale. Forgive my obtuseness in challenging something "almost true by definition" I guess.

"A population with aging has more diversity [...] compared to a similar population in which death is only due to famine, predators, disease, etc." Has this actually been studied? It seems a challenging question to answer.

"[T]here is abundant evidence that aging is no accident, that it has evolved via natural selection that explicitly favors aging." Where?

Furthermore it's not enough to argue that something would be favored by selection. It must have a significant coefficient of selection to evolve in the first place. Evolutionary storytellers usually neglect that part. Fact is, not very many animals die of aging. Captive animals often live many times as long as wild ones, even though they age the same. This means that the overall visibility of aging to selection is rather low and this would drive down the coefficient.

I'll leave off with a whimsical puzzle: where are the suicidal animals? Behavioral adaptation can happen extremely fast, as shown in domestication experiments, wild species that become much more wary of humans over just a few generations when hunted, etc. If aging were evolved, it is strange that the behavioral "implementation" is never chosen despite that there are many simple behaviors available that lead to death. Note that examples like bees do not resolve the dilemma, since they die for their very close kin, not to remove themselves for the good of the species.

Posted by: José at August 13, 2014 2:31 PM

@José: I meant that biology is up for the task in general: capable of it in the abstract as illustrated to various degrees from most to least in hydra, lobsters, mole rats, etc. The question is then why not in humans or most other complex organisms. In the past I've drawn the dividing line between hydras and more complex organisms on the basis of possessing a complex nervous system, since once you have one of those extreme regeneration is no longer possible, but there are other lines to be drawn between, say, lobsters and people, regarding why things are the way they are.

Posted by: Reason at August 13, 2014 4:22 PM

Seems more likely that we age because damage accumulates that our bodies do not bother to repair well (because in the wild we'd be dead of other causes first).

Posted by: Jim at August 13, 2014 5:12 PM

I was thinking the same thing as José when I read this post: the main message I got from these researchers was that aging is an evolved mechanism that improves reproductive fitness (by shortening our lifespans and making us more evolvable, though José points out this is not a well grounded position) rather than a degenerative process that evolution has not produced sufficient maintenance processes to cope with (because reproduction occurs before advanced aging sets in and maybe the caloric cost of investing in better maintenance systems outweighs the cost of producing new organisms). This is in tension with the view that aging is not an evolved adaptation which has often been stated here.

Posted by: gheme at August 14, 2014 11:00 AM

@gheme: Correct, the fellow quoted in the post above is very much in the programmed aging camp if you take a look through his other publications. Not a set of theories I agree with, but in many cases evolutionary discussions are largely focused on outcomes rather than on how those outcomes are implemented under the hood. A programmed aging and aging as damaged theorist can discuss a wide range of evolutionary theories without ever touching on their (rather important) points of difference.

Posted by: Reason at August 14, 2014 12:13 PM

If aging is genetically programmed, that means we can't change it. Repairing the damage won't be enough. So we better hope the programmed aging guys are wrong, because if they aren't, there's no way to stop aging. Not within a single adult human, anyway. Maybe it'll be possible to genetically engineer future generations from the zygote, but as for currently-alive humans....nope.

Posted by: Dinosaurus at August 14, 2014 3:44 PM

@Dinosaurus: Yes, that's about the sum of it. Fortunately, from my view of the field, the evidence strongly favors aging as damage. The regulatory situation strongly favors using drugs to tinker with gene expression, protein levels, and epigenetic patterns, however, which is why we so much work that fits with a programmed aging view of the world even though the overwhelming majority of researchers are in the aging as damage camp. It is something of a ridiculous state of affairs.

Posted by: Reason at August 14, 2014 4:39 PM

Those of you participating in this discussion may appreciate this.

Posted by: Michael-2 at August 14, 2014 10:22 PM

Reason, Dinosaurus: no no, you are completely wrong that "If aging is genetically programmed, that means we can't change it". On the contrary, it would be completely fantastic if aging were genetically programmed, because then we could postpone aging by breaking a program, which tends to be a far easier thing to do than augmenting what we naturally have (which is what SENS is all about). That's why I spend so much time debating with Josh and his ilk: even though I'm certain they are wrong, the faint possibility that I've overlooked something is enough to be worth giving them the utmost benefit of the doubt, because if they were right we could start saving lives a lot sooner than SENS will. Note that being programmed does not in any way mean not being mediated by damage accumulation - it just means that the accumulation is driven actively by gene expression, rather than by the gaps in our anti-aging arsenal. Of course this is kinda circular, since my definition of damage is "any change to the molecular or cellular composition or structure of the body that occurs cumulatively throughout life and eventually impairs the body's normal function" - but that's fine, because the clock that the program (if there is one) runs to still needs to be manifest in some physical form or other.

Posted by: Aubrey de Grey at August 19, 2014 11:49 AM

Evolution only selects for reproduction and once we're past reproductive viability it has no mechanism to select for our survival. Age may hold some utility for the mayfly, but hasn't really had a mechanism to be selected for hominids because for most of our evolutionary history hominids didn't survive for long enough for it to be an issue. If we had survived for long enough for our evolution to be influenced by old age, there would likely be more mechanisms to protect us from it due to the positive benefit in child rearing.

Posted by: Janos at February 17, 2015 5:59 AM
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