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The near equality $m_u\simeq m_d$ makes the QCD Lagrangian approximately invariant under the isospin group $SU(2)$ acting on $(u,d)$. Treating $m_u,m_d,m_s$ as small compared with the hadronic scale enlarges this to approximate $SU(3)$ flavor symmetry, whose hadron multiplets form the eightfold way. The observed multiplet pattern motivated the quark model, while the need for a hidden three-valued quantum number and a consistent strong dynamics led to color and QCD.

QCD explains both symmetries because its gluon coupling is flavor blind; only the quark-mass matrix breaks flavor symmetry. There is no useful larger light-flavor symmetry because $m_c,m_b,m_t\gg {\Lambda }_{\mathrm{QCD}}$. Their mass terms break $SU(N_f)$ too strongly for hadrons containing them to join approximately degenerate extensions of the light-quark multiplets.