On the Evolution of Language Models without Labels: Majority Drives Selection, Novelty Promotes Variation

Large language models (LLMs) can improve through reinforcement learning with verifiable rewards (RLVR), but many settings lack reference answers or external verifiers. Label-free self-improvement instead constructs training signals from the model's own outputs. Because the same policy generates both the candidate responses and the signals derived from them, each update also changes the candidates available for future training, creating a self-reinforcing feedback loop. We identify a failure mode of this loop, which we call selection-induced diversity collapse: repeated self-selection can progressively narrow the candidate pool and make alternative correct solutions harder to sample. To address this problem, we propose Evol-RL, which balances selection and variation. Majority voting provides a stable selection signal, while embedding-based novelty refines credit among responses with the same selection status; entropy regularization and asymmetric clipping further support this process by maintaining diverse rollouts and preserving rare, high-advantage updates. Across multiple training distributions, model scales, backbones, and matched label-free baselines, Evol-RL consistently improves pass@n while generally maintaining or improving pass@1. For example, when trained without labels on AIME24, Qwen3-4B-Base improves over TTRL on held-out AIME25 from 4.6% to 17.1% in pass@1 and from 18.5% to 42.0% in pass@16. Training analyses further show that when the current majority is wrong, Evol-RL better preserves alternative correct solutions. Code is available at https://anonymous.4open.science/r/EVOL-RL.