 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 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.