  We examine the evolution of assembly bias using a semi-analytical model of
galaxy formation implemented in the Millennium-WMAP7 N-body simulation. We
consider fixed number density galaxy samples ranked by stellar mass or star
formation rate. We investigate how the clustering of haloes and their galaxy
content depend on halo formation time and concentration, and how these
relationships evolve with redshift. At $z=0$ the dependences of halo clustering
on halo concentration and formation time are similar. However, at higher
redshift, halo assembly bias weakens for haloes selected by age, and reverses
and increases for haloes selected by concentration. The variation of the halo
occupation with concentration and formation time is also similar at $z=0$ and
changes at higher redshifts. In this case, the occupancy variation with halo
age stays mostly constant with redshift but decreases for concentration.
Finally, we look at the evolution of assembly bias reflected in the galaxy
distribution by examining the galaxy correlation functions relative to those of
shuffled galaxy samples which remove the occupancy variation. This correlation
functions ratio monotonically decreases with larger redshift and for lower
number density samples, going below unity in some cases, leading to reduced
galaxy clustering. While the halo occupation functions themselves vary, the
assembly bias trends are similar whether selecting galaxies by stellar mass or
star formation rate. Our results provide further insight into the origin and
evolution of assembly bias. Our extensive occupation function measurements and
fits are publicly available and can be used to create realistic mock
catalogues.
