  The light elements D, \he3, \he4, and \li7 are produced in big bang
nucleosynthesis and undergo changes in their abundances due to galactic
processing. Since one may observe most of these elements only in contemporary
environments, knowledge of the intervening evolution is necessary for
determining the observational constraints on primordial nucleosynthesis.
Chemical and stellar evolution model dependences in light element evolution are
systematically investigated via a comparison of 1460 possible chemical
evolution scenarios and of stellar nucleosynthesis yields, all of which have
been selected to fit solar neighborhood C, N, O, and Fe abundances as well as
the observed local gas density and gas mass fraction. The light element
evolution and solar system yields in these models are found to span a wide
range, explicitly demonstrating the model dependence. The range of model
dependence for D, \he3, and \he4 solar abundances is calculated, and its
sensitivity to the heavy element constraints is noted. The chemical evolution
contribution to the uncertainty in the observed primordial light element
abundances is estimated, and the effects of this uncertainty on big bang
nucleosynthesis results are discussed. The predictions for the light elements
are found to be correlated; the extent and physical origin of these
correlations is discussed. D and \he3 evolution is found to have significant
model dependence, however, the dominant factor determining their solar and
interstellar abundances is their primordial abundance. In addition, \he3 is
found to be very sensitive to the details of processing in low mass stars. \he4
yields are shown to be very model dependent; in particular, both the
introduction of mass loss and the possibly very high \he4 stellar yields in the
