Effect of binary stellar pathways on the evolution of the Q(H I)/Q(He II) ratios attained by stellar population synthesis models at low metallicities. The tracks show the ionizing photon flux ratio with time for the BPASS and BC03-2016 models at Z = 1 × 10⁻⁴ and the Yggdrasil model at Z = 4 × 10⁻⁴. The region spanned by the observational constraints in Fig. 13 is shaded. While the single-star evolution models begin life bluer at these metallicities, binary pathways generate hard ionizing flux at later ages.