Each variant is characterised by the net fecundity function V i . We assume that mutants are defined by V Ã (x)~V (x) 1zd(x) where d(x) is monotonic in x. As a consequence, mutants arise from translation on the function V (x) (corresponding to a change in the age of infector) or from a re-scaling of V (x) (corresponding to an increase or decrease in the net fecundity function). Monotonicity is imposed to preclude net-fecundity profiles that are large at early and late stages and low at intermediate stages.