In reply to Al Evans:
Hi,
that has been studied extensively, and the theory behind it is pretty much understood. This makes it less of a discussion point, unless some extreme example is found, e.g. the recently discovered Malegasy chameleons that never meet the next generation: Hatch, mate, lay eggs, die, (wait through dry season), hatch,.... Essentially, one generation of animals is born every year that lives for about three months, and for 9 months you will only find eggs.
To be brief, life span is over evolutionary time selected to maximise the number of ones offspring in the gene pool of the later generations. This is easy to see in extreme cases like salmon. Even for an adult salmon, the chances of making the trip to sea and back for a second time are marginal. It is therefore much more efficient to allocate all resources into making egg or sperm, and die, rather than survive mating season and likely be eaten before getting a second shot.
Female Orcas on the other hand are long lived, livig for several decades even after menopause when they cannot have offspring of their own anymore. Here it was shown that there is a strong effect on the survival of their sons, presumably through better hunting success of larger pods. Living longer rewards the individual with more grandchildren, and is thus fixed in the population.
This of course comes on top of an older evolutionary commitment towards sexual reproduction. We are not immortal but only our germline is, which e.g. helps with getting rid of mutational load.
Cheers,
CB