Selection Shadow Theory
Peter Medawar’s 1952 theory explains aging through the declining force of natural selection with age. The basic insight: harmful mutations that only cause problems late in life can accumulate because selection can’t “see” them.
The core mechanism
Most animals die from external causes - predation, disease, environmental hazards - before they get old enough for aging to matter. This creates a “selection shadow” where mutations affecting only late-life survival face weak or no selective pressure.
A mutation that kills you at 80 has little evolutionary impact if most of your species dies by 40 anyway. Natural selection optimizes for reproductive success,
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