The selection shadow is a concept involved with the evolutionary theories of aging that states that

selection pressures Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

on an individual decrease as an individual ages and passes

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definitio ...

, resulting in a "shadow" of time where selective fitness is not considered. Over generations, this results in maladaptive mutations that accumulate later in life due to aging being non-adaptive toward reproductive fitness. The concept was first worked out by

J. B. S. Haldane John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolog ...

and

Peter Medawar Sir Peter Brian Medawar (; 28 February 1915 – 2 October 1987) was a Brazilian-British biologist and writer, whose works on graft rejection and the discovery of acquired immune tolerance have been fundamental to the medical practice of tissue ...

in the 1940s, with Medawar creating the first

graphical model A graphical model or probabilistic graphical model (PGM) or structured probabilistic model is a probabilistic model for which a Graph (discrete mathematics), graph expresses the conditional dependence structure between random variables. They are ...

Model

The model developed by Medawar states that due to the dangerous conditions and pressures from the environment, including predators and diseases, most individuals in the wild die not long after sexual maturity. Therefore, there is a low probability for individuals to survive to an advanced age and suffer the effects related to

aging Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...

. In conjunction with this, the effects of natural selection decrease as age increases, so that later individual performance is ignored by selection forces. This results in beneficial

mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

not being selected for if they only have a positive result later in life, along with later in life deleterious mutations not being selected against. Due to the fitness of an individual not being affected once it is past its reproductive prime, later mutations and effects are considered to be in the "shadow" of selection. This concept would later be adapted into Medawar's 1952 mutation accumulation hypothesis, which was itself expanded upon by George C. Williams in his 1957

antagonistic pleiotropy hypothesis The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls for more than one phenotypic trait in an organism. Antagoni ...

. A classical requirement and constraint of the model is that the number of individuals within a population that live to reach

senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...

must be small in number. If this is not true for a population, then the effects of old age will not be under a selection shadow and instead affect adaptation and evolution of the population as a whole. At the same time, however, this requirement has been challenged by increasing evidence of senescence being more common in wild populations than previously expected, especially among birds and mammals, while the effects of the selection shadow remain present.

Medawar's Test Tube model

Medawar developed a theoretical model to demonstrate his thought process which explained that most animals will die before aging will be the ultimate cause for death in that animal. This would be from environmental factors such as large storms, drought, and fires, and predation. Medawar wanted to demonstrate this possibility by using test tubes to get his point across. The test tubes would be used to represent a population of species. If one of these test tubes were to theoretically break, this would represent an individual animal dying. Randomly, test tubes would then be broken in the population to keep his model realistic. The broken test tubes would be replaced with a new one, which represents a new animal being born into the population. Over time, the model showed that test tubes over a certain age would decline in the population as new test tubes were put in. The overall results in Medawar’s thought model demonstrated an exponential decline in the survivor curve which resulted in the population having a half life. The amount of older animals, or test tubes in the population would then be harder to maintain and ultimately die. Medawar created this model to ultimately explain what would realistically happen in actual life.

Criticism

Some scientists, however, have criticized the idea of aging being non-adaptive, instead adopting the theory of "

death by design ''Death by Design'' is a 1943 British mystery film directed by Germain Burger and starring John Longden, Wally Patch and Barbara James. Cast * John Longden as Inspector Slade * Wally Patch Walter Sydney Vinnicombe (26 September 1888 – ...

". This theory follows the work of

August Weismann August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Cha ...

, which states that aging specifically evolved as an

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...

, and disagrees with Medawar's model as a perceived oversimplification of the impact older organisms have on evolution. It is also claimed that older organisms have a higher reproductive capacity due to being better fit in order to reach their age, rather than their capacity being equal as in Medawar's calculations.

References

{{reflist Life extension Evolutionary biology Senescence Evolutionary theories of biological ageing