  Motivated by four-dimensional superstring models, we consider the possibility
of treating the Yukawa couplings of the Minimal Supersymmetric Standard Model
(MSSM) as dynamical variables of the effective theory at the electroweak scale.
Assuming bottom-tau unification, we concentrate on the top and bottom Yukawa
couplings, and find that minimizing the effective potential drives them close
to an effective infrared fixed line. Requiring an acceptable bottom-top mass
ratio leads in principle to an additional constraint on the MSSM parameter
space. As a by-product, we give new approximate analytical solutions of the
renormalization group equations for the MSSM parameters.
