  On the basis of Quantum Monte Carlo simulations of the two-dimensional
Hubbard model which cover the doping range from the under- to the over-doped
regime, we find that the single-particle spectral weight $A (\vec k,\omega)$
qualitatively reproduces both the momentum ($d_{x^2-y^2}$--symmetry) and doping
dependence of the pseudogap as found in photoemission experiments. The drastic
doping dependence of the spin response $\chi_{s} (\vec q,\omega)$ which is
sharp in both $\vec q (\approx(\pi,\pi))$ and $\omega$ in the under-doped
regime but broad and structureless otherwise, identifies remnants of the
antiferromagnetic order as the driving mechanism behind the pseudogap and its
evolution with doping.
