  We use very deep (R_lim=27) UGRI imaging to study the evolution of the faint
end of the UV-selected galaxy luminosity function from z~4 to z~2. We find that
the luminosity function evolves with time and that this evolution is
differential with luminosity: the number of sub-L* galaxies increases from z~4
to z~3 by at least a factor of 2.3, while the bright end of the LF remains
unchanged. Potential systematic biases restrict our ability to draw strong
conclusions at lower redshifts, z~2, but we can say that the number density of
sub-L* galaxies at z~2.2 is at least as high as it is at z~3. Turning to the UV
luminosity density of the Universe, we find that the luminosity density starts
dropping with increasing redshift already beginning at z=3 (earlier than
recently thought - Steidel et al. 1999) and that this drop is dominated by the
same sub-L* galaxies that dominate the evolution of the LF. This differential
evolution of the luminosity function suggests that differentially comparing key
diagnostics of dust, stellar populations, etc. as a function of z and L should
let us isolate the key mechanisms that drive galaxy evolution at high redshift.
