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Evolution: Why Do We Age And Die Rather Than Live Forever?


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#31 Cameron

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Posted 11 September 2010 - 08:46 AM

Elaboration:



My take is the ideas in multilevel selection. At first some molecules that replicate compete, the best replicator would normally win, but a group of molecules each slowly becoming slightly more specialized at its task will outcompete as a group a single molecule replicator. Once these groups of replicating entities establish dominance, these groups will still experience decreasing complexity(entropy, decay ), a few will acquire further specialization, but will not increase in complexity, but it is not until selection of some cells that happen to have acquired cooperative traits allows higher levels of organization to take place. The accidental forming of a group of slightly cooperative units at one level, leads to selection at a higher level, creating a complexity ratchet.

Some within these groups of cells will in turn accidentally create even larger cooperative groups, which will have the ability to persist in the environment and fend off simpler replicators. It can be seen that although simpler structures of a lower level tend to have replicative advantage, a more complex unit with a lower level of replication can persist in the environment. This creates a pool where even higher level cooperative structures can spontaneously randomly emerge.

Structures enabling viable higher levels of cooperative organization may emerge, even if they are highly costly in energy terms, and even if they make essential things like reproduction(birth) difficult and dangerous.

It can be seen that a cell is nothing more than a ship for a group of genes, a gene that increases its individual fitness can some of the time do this at the cost of the group, i.e. meiotic drive, transposons, etc. We see this too in computers with genetic algorithms, were if a small subset's progeny comprise too much of the next generation, diversity can be lost and things can settle in suboptimal solutions. In biology with, artificial selection too strongly favoring a small subset of the group, can lead to inbreeding and decrease the fitness of the whole group or population. This inbreeding, and reduction of diversity can be seen as identical to the outcome of an individual becoming too fit and able to it and its progeny outcompete too much the other members of the species.

It can be seen that at each higher level of organization it is necessary to keep the components at the next lower level to a certain extent in check. Take for example a small isolated tribe, if the chief is too successful, too fit, by successfully imposing a rule that only he can mate with any of the tribes females, it can be seen that if this rule holds for generations the tribe will simply cease to exist due to inbreeding. In a society men are kept in check by necessary laws(laws that actually reduce the reproductive fitness of at least some individuals... e.g. freely unethically raping lone women without punishment, would tend to lead to more progeny.). In a multicellular organism, cells have to be kept in check, and within cells genes have to be kept in check.

In a group of cells it can be seen that mutations that increase the fitness of an individual cell can do so at the cost of the group of cells, cancer. Mutations that allow more copies of a particular gene to be made(by it copying itself multiple times across the genome or skewing meiosis), will cost the group of genes more in terms of resources, diversity and reproductive fitness.

It is likely sex itself is a form of segregating genes into various smaller pools, were the most cooperative(altruistic) pools(individuals were the gametes fall) outcompete the less cooperative ones. The cost of 50% of the genes being lost per individual progeny a nice price to pay to allow group selection of various random alternatives to play out, as it is impossible to foretell the best combination or most cooperative gene group. This idea can also be seen in economy were convergence in a particular market of many corporations into a single monopoly is detrimental to the market, a smaller group, maybe even an oligopoly is preferable to a monopoly, competition between groups increases market efficiency and yields better results for consumers, in a similar manner it is likely keeping groups of genes in smaller competing pools also allows for better outcomes.

As for overall replicative fitness, in a sense an organism from a 'higher level of complexity' can have lower and slower replicative functions while still persisting in the environment. A cockroach replicates alot, and often even competes for the same food sources as humans, yet it is nothing more than a pest.

Thus, even if one did take group selectionism seriously (which almost no evolutionary biologists do nowadays) there'd be nothing for selection to act on!



The problem is that this made even talking of superorganisms, or talking of multicellular organisms as groups taboo, hindering things. Why should one level of a hierarchy be privileged? The level of a single multicellular organism? When it is seen that a simpler group of molecules gave way to a more complex group of molecules a cell and a group of cells gave way to a more complex group of cells, which gave way to groups of multicellular organisms that cooperated ans specialized, which gave way to even higher level groups.

Multilevel selection, the more recent ideas on group selection, make lots of sense. A lot of the arguments limiting group selection were based on simplifications that have been shown not to hold in reality. Even things like kin selection have been cast in the light of multilevel selection.

Which I would say suggests lots of things, it seems to suggests that selection at the level of a group at a particular level(cooperating molecules, cells, multicellular organisms, society, societies, etc), opens the door to the possibility of increasing complexity, forming a sort of complexity ratchet.

Through the passage of time and through the increases in complexity, it is seen that group selection moves higher, becoming ever more impactful at higher levels. At first it acts on groups of molecules, then groups of cells, then groups of multicellular organisms, then societies, etc. In fact in organisms like humans it doesn't matter much if an individual of a particular ethnic group is the overall 'fittest' human on earth, as another group can do genocide and wipe his group off the face of the earth along with his kin, genes and children all gone up in smoke.

Obviously selection acted on something, one group remains the other is gone, the group that won may even have some serious genetic disease prevalent in their population due to some pop. bottleneck in their past, thus likely being less fit than the group that was eliminated.

I think it is very difficult to consider this problem because there are no good examples of a complex organism with extreme longevity.



It's not a solid example. But there are suggestions that some whales may actually exhibit negligible senescence, aka, biological immortality. Interesting being at the top of the food chain, and having large quantities of matter flowing through them means they're exposed to alot of naturally mutagenic compounds naturally present in the environment, yet some suggest nearly two centuries without dying of cancer, despite having orders of magnitude more cells than humans(note more cells equals more replications and thus even more additional chances for error and cancer, which compound with the environmental toxin exposure from being at the top of the food chain and consuming large quantities of food.).

In humans we see that some single cells can last for over a century despite extremely high levels of metabolic activity(neurons, the metabolic activity is supposedly mostly related to their signalling function,), while other dividing cells experience problems, While other cells do need to divide, it does not follow that division inevitably leads to decay(clonal organisms).... nor cancer. There are individuals who eat meat that's burnt black and heavily smoke, yet reach 90+ years without dying of cancer, iirc.It seems extremely unlikely that no cell turned cancerous nor any eventually malignant. Much more likely that some did and the immune system fought it off.

As for senescent cells, it is very suspicious that these cells become seemingly so quickly dysfunctional and start to accumulate.... stopping division shouldn't by itself cause aberrancy, neurons stopped dividing and many can remain highly functional for over a century. Not sure but IIRC, telomerase seems to restore functionality or the appearance of it, if this is true(If I recalled correctly) it seems that they[senescent] have the capacity to remain highly functional but somehow being in a non dividing state causes them to misbehave .

Edited by Cameron, 11 September 2010 - 08:54 AM.





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