  Over the past 10 Gyr, star-forming galaxies have changed dramatically, from
clumpy and gas rich, to rather quiescent stellar-dominated disks with specific
star formation rates lower by factors of a few tens. We present a general
theoretical model for how this transition occurs, and what physical processes
drive it, making use of 1D axisymmetric thin disk simulations with an improved
version of the Gravitational Instability-Dominated Galaxy Evolution Tool
(GIDGET) code. We show that at every radius galaxies tend to be in a slowly
evolving equilibrium state wherein new accretion is balanced by star formation,
galactic winds, and radial transport of gas through the disk by gravitational
instability (GI) -driven torques. The gas surface density profile is determined
by which of these terms are in balance at a given radius - direct accretion is
balanced by star formation and galactic winds near galactic centers, and by
transport at larger radii. We predict that galaxies undergo a smooth transition
from a violent disk instability phase to secular evolution. This model provides
a natural explanation for the high velocity dispersions and large clumps in z~2
galaxies, the growth and subsequent quenching of bulges, and features of the
neutral gas profiles of local spiral galaxies.
