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Williams' idea, now known as the antagonistic pleiotropy theory of aging, remains the prevailing evolutionary explanation of senescence, the process of becoming old or aging.

In genetics, the concept of pleiotropy posits that a single mutation can influence multiple traits.

The idea that the same mutation can be both beneficial and harmful, depending on the situation, is known as antagonistic pleiotropy and was proposed by Williams to underlie the origin of aging in a paper titled "Pleiotropy, natural selection, and the evolution of senescence."

"The antagonistic pleiotropy hypothesis predicts that most mutations that increase reproduction but reduce lifespan have larger fitness advantages than disadvantages so are selectively favored," Zhang said.

It’s a classic case of antagonistic pleiotropy, where a process that is beneficial earlier in life becomes harmful later on.