 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.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.