Anything has a lifespan. Unless there is some input of energy to maintain an equilibrium (e.g. maintain the integrity of DNA) things will tend towards disorder (according to the second law of theromdynamics). I guess you could say that death will result when there is not enough input of energy to maintain the integrity of the metabolic functions in an organism. This could happen when, for instance, not enough food is available or when energy is used to produce offspring instead (See my message #18 for link to an article that talks about the disposable some theory).
DNA/RNA simply replicate themselves correct? How is death a factor here? Multi cellular organisms seek to maintian their reaction. This is not a factor in the case of DNA/RNA. At some point the individual organism began to exist for itself beyond replication. What was the advantage of this and was this when death as we understand it became a factor?
(The following piece will only deal with multucellular organisms)
You asked earlier in message #23 "How does "survival of the fittest" require death?". It doesn't. You can always think of it this way: While "the purpose of a multicellular organism might seem to be to make more multicellular organisms", instead think of it as "the role of gametes (i.e. egg and sperm) is to make more gametes through the vehicle of a multicellular organism". In this sense, multicellular organisms are just as immortal as prokaryotes are (although even they seem to age to an extent).
This is an interesting view. If you are to think in these terms then you should expand this to the entire biomass as a whole. Immortality does not scientifically apply. Neither does implying a specific "purpose" to any multicellular organisms existence. Death is a fact though. Again.....countless living things seek to maintain themselves as individuals far beyond replication. I ask again...the big wind up but how came about the consistent wind down? Was it natures way of balancing an equation?