Non-scalable genome fidelity constraints drive the evolution of sexual dimorphism

The evolutionary origin of sexual dimorphism is classically explained by energetic trade-offs between gamete size and number interacting with fertilization ecology. These models, however, treat genome fidelity as a passive or metabolically subsumed property. Here we show that informational constraints alone - specifically the limited scalability of molecular machinery required for genome replication, repair, and chromatin maintenance are sufficient to destabilize symmetric reproductive strategies and generate stable reproductive dimorphism. We introduce the Molecular Fidelity-Encounter Competition (MF–EC) model, an Adaptive Dynamics framework in which reproductive strategies evolve along a heritable axis representing parental reproductive deployment. Encounter success increases concavely with deployment, whereas per-gamete molecular fidelity declines convexly due to dilution of finite parental fidelity budgets. Beyond a biologically realistic fidelity-dilution threshold, the unique evolutionary equilibrium becomes convergence-stable but invasion-unstable, enforcing disruptive evolutionary branching. Invasion fitness analysis and replicator dynamics demonstrate that the adaptive landscape supports exactly two mutually invasion-resistant strategies, while all intermediate strategies are eliminated. This framework provides a principled explanation for the near-universality of binary sexual systems and identifies informational constraints as a fundamental, non-energetic driver of sexual dimorphism.