e to build a model in which the effect of consumption is greatest on age classes with high fertility. Marginal fertility and survival become Ti, = —m(c, K) (11) ~ Powe. Pe = —P(e,k) (12) and the MRS in fitness is , pr (4) 1 —v)POy@ (2) MRS» = (< ) ye +(1— 7)PYv K (13) r ym + (1 = y)PQy@ ] \ 6 where ‘y