Time evolution of the Ks-band mass-to-light ratio, M_⋆/L_K, for stellar populations with a fixed canonical (Kroupa 2001) IMF and different metallicities denoted in the upper left corner of each panel. Red solid curves are SSPs formed at time zero. The sharp drop in the SSP models of M_⋆/L_K at t ≈ 2 × 10⁹ yr is caused by a significant increase in the number of AGB stars. The blue dashed curves are stellar populations with the same fixed metallicity but a constant SFR over time, demonstrating a smooth curve of lower M_⋆/L_K value due to the luminosity contribution from young stars. The exact mass of the SSP or the SFR for the constant SFR model does not affect the IMF by definition, and therefore, does not affect the M_⋆/L_K evolution curve.