Strong selection for what’s within reproductive span
Natural selection carefully weeds out all genes that might allow damage to the body before or during reproduction. It does so by killing or lowering the reproductive success of all individuals that express such genes in youth. All the rest reproduce. But natural selection cannot weed out genes that damage the body in post-reproductive old age, because there is no reproduction of the successful in old age. Take Dunnet’s fulmar, for instance. The reason it lives far longer than a mouse is because in the life of the fulmar there is no equivalent of the cat and the owl: no natural predators. A mouse is unlikely to make it past three years of age, so genes that damage four-year-old mouse bodies are under virtually no selection to die out. Fulmars are very likely to be around to breed at twenty, so genes that damage twenty-year-old fulmar bodies are still being ruthlessly weeded out.
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