The selection shadow is a concept involved with the evolutionary theories of ageing that states that selection pressures on an individual decrease as an individual ages and passes sexual maturity, 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 and Peter Medawar in the 1940s, with Medawar creating the first graphical 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. 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 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.
A classical requirement and constraint of the model is that the number of individuals within a population that live to reach senescence 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.
Some scientists, however, have criticized the idea of aging being non-adaptive, instead adopting the theory of "Death by Design". This theory follows the work of August Weismann, which states that aging specifically evolved as an adaptation, 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.
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