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, which happens early in most species' lifespans. Once reproduction is complete, selection pressure drops dramatically.
Mathematical formalization¶
William Hamilton later formalized this insight mathematically, showing that the force of selection declines exponentially with age in populations with overlapping generations. The strength of selection on traits expressed at age x is proportional to the reproductive value at that age.
Mutation accumulation¶
This leads to mutation accumulation - deleterious mutations that affect late-life performance can drift to high frequencies because they face weak selection. Over evolutionary time, populations accumulate a burden of age-specific harmful mutations.
The theory predicts that aging should start around the age when most individuals would naturally die in the wild, which roughly matches observed patterns across species.
Modern relevance¶
Medawar's insight remains fundamental to aging biology. It explains why we don't see strong selection for longevity mechanisms and why aging appears universal among complex organisms.
The theory also predicts that extending lifespan in protected environments (modern medicine, captivity) should reveal previously hidden genetic damage - which matches observed patterns of age-related disease.
Relationship to other theories¶
Medawar's work laid the foundation for later evolutionary theories of aging, particularly antagonistic pleiotropy and disposable soma theory. All share the insight that aging results from natural selection's focus on early-life performance rather than longevity.
The selection shadow explains why evolution tolerates aging: by the time harmful effects appear, natural selection has already moved on to the next generation.