  For atoms and crystals with an ideal symmetry, the optical selection rules
for electronic transitions are well covered in physics textbooks. However, in
studies of material systems one often encounters systems with a weakly
distorted symmetry. Insight and intuition for how optical selection rules
change when an ideal symmetry is gradually distorted is, nevertheless, little
addressed in literature. We present here a detailed analysis of how a gradual
symmetry distortion leads to a complete alteration of optical selection rules.
As a model system, we consider the transitions between $1s$ and $2p$ sublevels
of the hydrogen atom, which get distorted by placing charged particles in its
environment. Upon increasing the distortion, part of the optical selection
rules evolve from circular via elliptical to linear character, with an
associated evolution between allowed and forbidden transitions. Our
presentation combines an analytical approach with quantitative results from
numerical simulations, thus providing insight in how the evolution occurs as a
function of the strength of the distortion.
