  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.
