Darwinian evolution optimizes a lot of system's degrees of freedom, among
other things (localized) mutation rates and generation durations. (It also
is a fount of contraintuitive nonlinearities, but that's another story).
The trivial argument is that immortal individuals are almost instantly
outperformed, especially in a Red Queen environment (you have to run like
mad just to stay in the same place, and twice as fast if you want to get
to somewhere else).
It should be in a FAQ somewhere.
ciao,
'gene