A solvable senescence model showing a mortality plateau
  We present some analytic results for the steady states of the Penna model of sen escence, generalised to allow genetically identical individuals to die at differ ent ages via an arbitrary survival function. Modelling this with a Fermi functio n (of modest width) we obtain a clear mortality plateau late in life: something that has so far eluded explanation within such mutation accumulation models. This suggests that factors causing variable mortality withi n genetically identical subpopulations, which include environmental effects, may be essential to understanding the mortality plateau seen in many real species. 
J.B.Coe, Y.Mao, M.E.Cates