The clustering of galaxies relative to the underlying mass distribution dec
lines with cosmic time for three reasons. First, nonlinear peaks become les
s rare events as the density field evolves. Second, the densest regions sto
p forming new galaxies because their gas becomes too hot to cool and collap
se. Third, after galaxies form, they are subject to the same gravitational
forces as the dark matter, and thus they tend to trace the dark matter dist
ribution more closely with time; in this sense, they are gravitationally "d
ebiased." In order to illustrate these effects, we perform a large-scale hy
drodynamic cosmological simulation of a A cold dark matter (Lambda CDM) mod
el with Omega(0) = 0.37 and examine the statistics of delta*(r, z), the den
sity held of recently formed galaxies at position r and redshift z. We find
that the bias of recently formed galaxies b* equivalent to [delta*(2)](1/2
)/[delta(2)](1/2), where delta is the mass overdensity, evolves from b* equ
ivalent to similar to 4.5 at z = 3 to b* similar to 1 at z = 0, on 8 h(-1)
Mpc comoving scales. The correlation coefficient r* equivalent to [delta de
lta*]/[delta(2)](1/2)[delta*(2)](1/2) evolves from r* similar to 0.9 at z =
3 to r* similar to 0.3 at z = 0. That is, as gas in the universe heats up
and prevents star formation, the star-forming galaxies become poorer tracer
s of the mass density field. We show that the linear continuity equation is
a good approximation for describing the gravitational debiasing, even on n
onlinear scales. The most interesting observational consequence of the simu
lations is that the linear regression of the galaxy formation density held
on the galaxy density held, b(*g)r(*g) = [delta*delta(g)]/[delta(g)(2)], ev
olves from about 0.9 at z = 1 to 0.35 at z = 0. Measuring this evolution, w
hich should be possible using the Sloan Digital Sky Survey, would place con
straints on models for galaxy formation. In addition, we evaluate the effec
ts of the evolution of galaxy formation on estimates of Omega from cluster
mass-to-light ratios, finding that while Omega(z) increases with z, the est
imate Omega(est)(z) actually decreases. This effect is due to the combinati
on of galaxy bias and the relative fading of cluster galaxies with respect
to held galaxies. Finally, these effects provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at reds
hift z similar to 0.5.