Just over 50% of the lifetime expected reproductive output of a newborn elf is concentrated into its first 700 years; even though it could in principle live for millennia, producing children at the same rate all the while, its odds of reproducing are best early in life.
I think your elf example is even more extreme than you make it out to be, at least when population size is increasing, since the offspring an individual produces early in life can produce their own descendants exponentially while the original is limited to constant fecundity. 50% of an elf’s expected direct reproductive output comes in the first 700 years (5.03 expected offspring). But the total number of descendants is exp((fecundity—mortality)*age) − 1, or about 544 expected descendants at 700 years. It’s clear that (in this extreme case where fecundity is an order of magnitude higher than mortality) the death of the original at this point would make effectively no difference to the number of descendants going forward.
This effect should also apply to oscillating population sizes even if there’s no net increase over time (probably a more typical situation in practice). However, it does not account for the infertile period at the beginning of an organism’s life.
To be slightly more realistic, by the time aging seriously limits the ability to reproduce in humans (say age 40) a human could easily have multiple offspring old enough to reproduce. So even if extrinsic mortality is zero, a mutation that causes death or infertility at age 40 is reducing marginal reproductive output only by a fairly small fraction, which could be easily outweighed if it also increases fertility when young.
I think your elf example is even more extreme than you make it out to be, at least when population size is increasing, since the offspring an individual produces early in life can produce their own descendants exponentially while the original is limited to constant fecundity. 50% of an elf’s expected direct reproductive output comes in the first 700 years (5.03 expected offspring). But the total number of descendants is exp((fecundity—mortality)*age) − 1, or about 544 expected descendants at 700 years. It’s clear that (in this extreme case where fecundity is an order of magnitude higher than mortality) the death of the original at this point would make effectively no difference to the number of descendants going forward.
This effect should also apply to oscillating population sizes even if there’s no net increase over time (probably a more typical situation in practice). However, it does not account for the infertile period at the beginning of an organism’s life.
To be slightly more realistic, by the time aging seriously limits the ability to reproduce in humans (say age 40) a human could easily have multiple offspring old enough to reproduce. So even if extrinsic mortality is zero, a mutation that causes death or infertility at age 40 is reducing marginal reproductive output only by a fairly small fraction, which could be easily outweighed if it also increases fertility when young.
I’m not sure about this. I have to think about it.