  This paper shows that differentiating the lifetimes of two phenotypes
independently from their fertility can lead to a qualitative change in the
equilibrium of a population: since survival and reproduction are distinct
functional aspects of an organism, this observation contributes to extend the
population-genetical characterisation of biological function. To support this
statement a mathematical relation is derived to link the lifetime ratio
T_1/T_2, which parametrizes the different survival ability of two phenotypes,
with population variables that quantify the amount of neutral variation
underlying a population's phenotypic distribution.
