Where does senescence collide?

The gradual decline in an organism's chances of survival and its rate of reproduction as it grows older, treated as an outcome that evolution permits because natural selection acts less forcefully on traits expressed late in life. Medawar's 1952 lecture An Unsolved Problem of Biology argued that harmful effects appearing after most reproduction has already occurred escape selection and accumulate, and Williams's 1957 Pleiotropy, natural selection, and the evolution of senescence added that genes favoured early in life can carry costs that are paid later. Here the referent is a population-level decline in fitness with age, read across whole organisms and their offspring, not the halting of one cell's division, the breakdown of a leaf, or the failure rate of a company.[1][2]

Unit

One organism's age-specific survival and fertility, summarised across a population

Recovery means

Not applicable

Measured by

Estimated from life tables and age-specific mortality and reproduction rates in a population

A stable state in which an individual cell permanently stops dividing while staying alive and metabolically active, reached after a limited number of divisions or in response to stress. Hayflick and Moorhead's 1961 The serial cultivation of human diploid cell strains showed that normal human cells divide only a bounded number of times in culture before they stop, and Hayflick's 1965 The limited in vitro lifetime of human diploid cell strains set out that ceiling in detail. The referent is the arrested condition of one cell, tracked by division counts and molecular markers, and it is separate from the aging of a whole organism, the death of a leaf, or the failure rate of an organization.[3][4]

Unit

The arrested state of a single cell after its capacity to divide runs out

Recovery means

Largely irreversible under normal conditions, though the state can be cleared or bypassed experimentally

Measured by

Assessed by counting population doublings and by markers such as senescence-associated beta-galactosidase and a halted cell cycle

The final, actively controlled phase of a leaf or other plant organ, in which the plant takes apart chlorophyll and other large molecules and moves the freed nutrients to seeds, roots, or new growth before the organ dies. Leopold's 1961 Senescence in Plant Development framed this shutdown as a programmed part of development rather than passive decay, and Lim, Kim, and Nam's 2007 Leaf senescence reviewed the genes and signals that run it. The referent is an orderly recycling program inside a living plant, judged by chlorophyll loss and nutrient export, unlike a cell that simply stops dividing, a population growing old, or a firm that fails.[5][6]

Unit

One leaf or organ moving through its programmed final phase

Recovery means

Can be delayed or partly reversed early on, but becomes irreversible as it proceeds

Measured by

Tracked by chlorophyll and protein loss, nutrient export, and expression of senescence-associated genes

The tendency for an organization's risk of failure to rise as it grows older, once its size is taken into account, sometimes called the liability of senescence or of aging. Barron, West, and Hannan's 1994 A time to grow and a time to die tracked New York City credit unions from 1914 to 1990 and found that older organizations could face higher, not lower, closure rates once growth and size were held constant. The referent is an age pattern in the death rate of whole organizations, estimated from founding and closure records, and it has nothing in common with a cell that stops dividing, a leaf that is taken apart, or the aging of a living body beyond the shared word.[7]

Unit

One organization's yearly hazard of closure at a given age

Recovery means

Not applicable

Measured by

Estimated by event-history models of closure against organizational age, size, and growth