  We use a halo occupation approach to connect MgII absorbers to dark matter
halos as a function of redshift. Using the model constructed in Tinker & Chen
(2008), we parameterize the conditional probability of an absorber of
equivalent width Wr being produced by a halo of mass M_h at a given redshift,
P(Wr|M_h,z). We constrain the free parameters of the model by matching the
observed statistics of MgII absorbers: the frequency function f(Wr), the
redshift evolution n(z), and the clustering bias b(Wr). The redshift evolution
of Wr>1 A absorbers increases from z=0.4 to z=2, while the total halo cross
section decreases monotonically with redshift. This discrepancy can only be
explained if the gaseous halos evolve with respect to their host halos. We make
predictions for the clustering bias of absorbers as a function of redshift
under different evolutionary scenarios, eg, the gas cross section per halo
evolves or the halo mass scale of absorbers changes. We demonstrate that the
relative contribution of these scenarios may be constrained by measurements of
absorber clustering at z>1 and z~0.1. If we further assume a
redshift-independent mass scale for efficient shock heating of halo gas of
Mcrit = 10^{11.5} Msol/h, absorber evolution is predominantly caused by a
changing halo mass scale of absorbers. Our model predicts that strong absorbers
always arise in ~Mcrit halos, independent of redshift, but the mass scale of
weak absorbers decreases by 2 dex from 0<z<2. Thus, the measured
anti-correlation of clustering bias and Wr should flatten by z~1.5.
