  We show how the shape evolution of the neutron-rich exotic Si and S isotopes
can be understood as a Jahn-Teller effect that comes in part from the
tensor-driven evolution of single-particle energies. The detailed calculations
we present are in excellent agreement with known experimental data, and we
point out of new features that should be explored in new experiments. Potential
energy surfaces are used to understand the shape evolutions. The sub-shell
closed nucleus, $^{42}$Si, is shown to be a perfect example of a strongly
oblate shape instead of a sphere through a robust Jahn-Teller mechanism. The
distribution of spectroscopic factors measured by $^{48}$Ca(e,e'p) experiment
is shown to be well described, providing a unique test on the tensor-driven
shell evolution.
