  Guided by considerations of flavour symmetry, we construct a set of exact
Standard Model (SM) renormalisation group evolution invariants which link quark
masses and mixing parameters. We examine their phenomenological implications
and infer a simple combination of Yukawa coupling matrices which plays a unique
role in the SM, suggesting a possible new insight into the observed spectrum of
quark masses. Our evolution invariants are readily generalised to the leptons
in the case of Dirac neutrinos, but do not appear to be relevant for either
quarks or leptons in the MSSM.
