  A natural example of evolution can be described by a time-dependent two
degrees-of-freedom Hamiltonian. We choose the case where initially the
Hamiltonian derives from a general cubic potential, the linearised system has
frequencies 1 and $\omega >0$. The time-dependence produces slow evolution to
discrete (mirror) symmetry in one of the degrees-of-freedom. This changes the
dynamics drastically depending on the frequency ratio $\omega$ and the
timescale of evolution. We analyse the cases $\omega = 1, 2, 3$ where the
ratio's 1,2 turn out to be the most interesting. In an initial phase we find 2
adiabatic invariants with changes near the end of evolution. A remarkable
feature is the vanishing and emergence of normal modes, stability changes and
strong changes of the velocity distribution in phase-space. The problem is
inspired by the dynamics of axisymmetric, rotating galaxies that evolve slowly
to mirror symmetry with respect to the galactic plane, the model formulation is
quite general.
