  The phenomenological symplectic model with a Davidson potential is used to
construct rotational states for a rare-earth nucleus with microscopic wave
functions. The energy levels and E2 transitions obatined are in remarkably
close agreement (to within a few percent) with those of the rotor model with
vibrational shape fluctuations that are adiabatically decoupled from the
rotational degrees of freedom. An analysis of the states in terms of their
SU(3) content shows that SU(3) is a very poor dynamical symmetry but an
excellent quasi-dynamical symmetry for the model. It is argued that such
quasi-dynamical symmetry can be expected for any Hamiltonian that reproduces
the observed low-energy properties of a well-deformed nucleus, whenever the
latter are well-described by the nuclear rotor model.
