  We present a mathematical simplification for the evolutionary dynamics of a
heritable trait within a two-sex population. This trait is assumed to control
the timing of sex-specific life-history events, such as the age of sexual
maturity and end of female fertility, and each sex has a distinct fitness
tradeoff associated with the trait. We provide a formula for the fitness
landscape of the population and show a natural extension of the result to an
arbitrary number of heritable traits. Our method can be viewed as a dynamical
systems generalisation of the Price Equation to include two sexes{, age
structure} and multiple traits. We use this formula to examine the effect of
grandmothering, whereby post-fertile females subsidize their daughter's
fertility by provisioning grandchildren. Grandmothering can drive a shift
towards higher sex ratios due to lengthening female post-fertile longevity,
leading to changes in fitness for both sexes. For males, increased longevity is
accompanied by a substantially longer fertile lifespan resulting in higher sex
ratios in the fertile ages. Our fitness landscapes show a net increase in
fitness for both males and females at longer lifespans, and as a result, we
find that grandmothering alone provides an evolutionary trajectory to higher
longevities.
