Debate and discussion of any biological questions not pertaining to a particular topic.
The shorter the lifespan of an individual the faster it can adapt to it's environment. Also a elder man/woman is another mouth to feed, this can harm the species as a whole as well as their own offspring. In addition to feeding the person, you risk over-population. And evolution even has a fail-safe hedge in over-population. Violence, cannibalism, asexuality, homosexuality, all increase in over-populated areas in mice studies.
Venik, I think you are wrong in some aspects: First, organisms do not care about overpopulation on the species level. Even though any single individual may have their own strategy to survive such scenario, any species on this planet will overpopulate all the areas and niches as long as they can - this has been seen dozens of times in the nature. For example, if a species with no natural predators moves to a new area (say, a previously isolated island), they will overpopulate so badly that they might go exting after destroying their habitat for example by eating all the plants. If species really behaved this way, they would agree to stop breeding when food or room starts to become scarce - but no species does this. They just fill it up the best they can. Even us humans, smart and all, keep doing this.
Therefore, it is unlikely that organisms would die of old age just to leave more room, because all the room will be taken anyway. More effective would be to stay alive and defend you offspring and your own genes, or simply keep populating - and some organisms do this. For example, many lower organisms do not "age", they simply divide and divide, and in theory part of any one bacterial cell is a part of something that lived millions of years ago and during that time, from this chain, nobody died of old age. It is true that many organisms get fewer children or proliferate slower if there is little room or nutrition available, but they do not stop because of a communal effort to avoid overpopulation. They do so just to ensure that their own genes have better chances to survive.
Second, evolution by the natural selection does not work for the good of the species, though this is a common misconception. For example, you or I do not die of old age just to leave more room for Jack and his son living next door. Heck, in many species the strongest child kills their weaker siblings - talk about the good of the species if you murder your own brothers and sisters! The only thing that matters is the multiplying of your own genes, not the genes of random members of your species. To the contrary, oftentimes the other members of one's own species can be their biggest competitors, because they hunt for the same food and fight for the same living spaces. Even with social organisms there is social cooperation only as long as both participants benefit, but even in this case the own genes come first.
The final, conclusive answer to the reason of aging is probably not yet know. It is not even known if it is "purposeful" or just a by-product of our metabolism and environment. But it is pretty clear, I think, that organisms do not die of old age just to leave room for others or to avoid overpopulation.
I'm pretty much siding Canalon in this: long life span becomes unsupported as soon as it ceases to help your genes (your offspring) to multiply. Thus, it could be so that we die of old age simply because, evolution-wise, there is no reason for us to live any longer.
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