Fisher's principle explains the 1:1 sex ratio as an evolutionary equilibrium rather than a fixed physiological constant. What follows from this distinction if a sudden disease halves the male population?

C. Parents producing sons gain higher relative fitness until parity returns

Equilibria rely on individual differential reproductive success rather than species-level rescue or permanent mutations. When males are scarce, individual parents yielding sons enjoy higher genetic representation, driving the ratio back to parity via negative frequency-dependent selection.