Safe Evolution with Circuit Anchors

In biological evolution, unconstrained mutation can lead to catastrophic outcomes: organisms may evolve enhanced capabilities while losing essential functions for survival. Nature's solution is textitdevelopmental constraints, where core regulatory genes remain anchored while peripheral genes adapt freely. We observe that current self-evolution algorithms for large language models lack analogous constraints. They optimize purely for capability, implicitly assuming safety will be preserved. Our experiments reveal this assumption to be dangerously wrong: models can textitmisevolve into powerful yet dangerous entities. Inspired by how Hox genes anchor body structure across 500 million years of evolution, we propose Circuit-Anchored Evolution (CAE). Using mechanistic interpretability, we identify a tiny textitsafety circuit, comprising less than 2% of model features, that causally mediates safety behaviors. We anchor this circuit during evolution, constraining it within a small displacement bound while allowing the remaining features to evolve freely. This mirrors the biological principle of textitevolvability with constraint: preserving what is essential while adapting what is peripheral. Experiments across 3 model families and two evolution algorithms demonstrate that CAE achieves superior safety preservation with minimal capability loss, substantially outperforming explicit reward-based constraints in both effectiveness and efficiency. Just as developmental constraints prevent biological evolution from producing nonviable organisms, circuit anchoring prevents model evolution from producing capable but dangerous systems.