The selection shadow is a concept Involved with the evolutionary theories of aging That states That selection Pressures one year individual Decrease year as individual ages and assists sexual maturity , resulting and in a “shadow” of Time Where selective fitness is not regarded. Over generations, this results in maladaptive mutations that accumulate later in life due to being non-adaptive to reproductive fitness.  The concept was first worked out by JBS Haldane and Peter Medawar in the 1940s, with Medawar creating the first graphical model . 
The model developed by Medawar states, which is one of the most dangerous conditions in the world, including predators and diseases. 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 decreases, so that later individual performance is ignored by selection forces.  This results in beneficial mutationsnot being selected for a positive outcome in the life of the disease. To to………………… 
This concept would later be adapted to Medawar’s 1952 mutation accumulation hypothesis , which was itself expanded upon by George C. Williams in his 1957 antagonistic pleiotropy hypothesis . 
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 rather affect adaptation and evolution of the population as a whole. At the same time, however, this requirement has been challenged by increasing the prevalence of common sclerosis in humans rather than in the general population, especially among birds and mammals, while the effects of the selection shadow remain present. 
Some scientists, however, have criticized the idea of aging as 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 disagreed with Medawar’s model as perceived oversimplification of impact. It is also claimed that older women have a higher reproductive fitness capacity than those who are more likely to reach their age, rather than being able to do so in Medawar’s calculations. 
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- Jump up^ Flatt, Thomas; Schmidt, Paul S. (July 18, 2009). “Integrating evolutionary and molecular genetics of aging” (PDF) . Biochimica and Biophysica Acta . Elsevier . 1790 (10): 951-962. doi : 10.1016 / j.bbagen.2009.07.010 . Retrieved May 20, 2014 .
- Jump up^ Turbill, Christopher; Ruf, Thomas (August 6, 2010). “Senescence Is More Important in the Natural Lives of Long-Than Short-Lived Mammals” . PLOS ONE . Public Library of Science . 5 (8). doi : 10.1371 / journal.pone.0012019 . PMC 2917356 . PMID 20700508 .
- Jump up^ Lachowicz, Miroslaw; Miękisz, Jacek (April 20, 2009). From Genetics to Mathematics . Singapore : World Scientific . p. 39. ISBN 9812837256 . Retrieved May 25, 2014 .