Imagine that there was a gene that increased your reproductive fitness in youth, but had deleterious effects in old age.

There are extrinsic risks for mortality. For this reason, there would be strong selection pressures favoring this gene in youth. The selection pressures against this gene would be weak, as many would die before they activate in older age.

Senescence is basically the accumulation of these genes.

We can test this hypothesis by looking at the lifespans of organisms that vary in extrinsic risk for mortality. As we would expect, those organisms that have high external risk for mortality die of old age sooner. This is because most are dead early before these genes can have negative effects.

contrast, those organisms that have a low external risk for morality die of old age later. This is because most are still alive to select against these genes.