  The evolution of a quantum system undergoing very frequent measurements takes
place in a proper subspace of the total Hilbert space (quantum Zeno effect).
When the measuring apparatus is included in the quantum description, the Zeno
effect becomes a pure consequence of the dynamics. We show that for continuous
measurement processes the quantum Zeno evolution derives from an adiabatic
theorem. The system is forced to evolve in a set of orthogonal subspaces of the
total Hilbert space and a dynamical superselection rule arises. The dynamical
properties of this evolution are investigated and several examples are
considered.
