Evolutionary Theories of Aging¶
Four major theories explain why organisms age from an evolutionary perspective. Each addresses different aspects of the same fundamental question: why hasn't natural selection eliminated aging?
The theories¶
Selection Shadow Theory - Peter Medawar (1952)
Natural selection weakens with age, allowing late-acting harmful mutations to accumulate. The foundation of modern aging theory.
Antagonistic Pleiotropy Theory - George Williams (1957)
Genes that benefit early life but harm later life can be strongly selected despite causing aging. Explains why aging mechanisms are often actively harmful.
Disposable Soma Theory - Tom Kirkwood (1977)
Organisms face resource allocation trade-offs between reproduction and somatic maintenance. Perfect maintenance is too expensive evolutionarily.
Defensive Degeneration Theory - Contemporary synthesis
Aging results from tumor suppressor mechanisms that prevent cancer. Many aging phenotypes are actively maintained cancer prevention systems.
How they fit together¶
These theories are complementary rather than competing:
- Selection shadow explains why evolution tolerates aging mechanisms
- Antagonistic pleiotropy explains why harmful mechanisms persist
- Disposable soma explains the resource constraints that necessitate trade-offs
- Defensive degeneration explains why aging mechanisms are often actively maintained rather than merely tolerated
Modern synthesis¶
Current aging research integrates insights from all four theories. The most sophisticated understanding sees aging as resulting from evolutionary optimization for reproductive success under constraint - limited resources, declining selection pressure with age, and the fundamental tension between cellular autonomy and multicellular cooperation.
None of these theories suggest aging is inevitable or unchangeable, but they do constrain what kinds of interventions are likely to work and what trade-offs we should expect.
Understanding why aging evolved helps explain what we can realistically do about it.