Biological Continued Pretraining Reshapes the Capability Profile of a Foundation Model Without Catastrophic Forgetting
It is widely assumed that continued pretraining (CPT) on a narrow, out-of-distribution corpus such as raw biological sequence must trade away a general-purpose model’s broad competence — the “alignment tax” or catastrophic-forgetting intuition. We test this directly, without any new training, by re-analyzing three checkpoints from a single lineage of a 26B-parameter Mixture-of-Experts model (Gemma-4-26B-A4B): the instruction-tuned base, the same model after biological CPT (8.7B tokens of DNA, protein, and biomedical text), and after subsequent supervised fine-tuning (SFT). Across three independent capability axes — general knowledge/reasoning (MMLU, ARC, HellaSwag), code generation (MBPP), and biomedical knowledge (BixBench) — we find that biological CPT does not degrade the model; it lifts it: MMLU +13 points, MBPP pass@1 nearly doubles (0.33 →0.63), and BixBench discrimination rises sharply (MCC 0.23 → 0.92). The single measured regression is truthfulness (TruthfulQA 8.8 points), a small and interpretable domain drift. A clean vocabulary-expansion ablation (< 0.4 pt on every general metric) confirms the gains are attributable to CPT, not tokenizer changes. Crucially, subsequent SFT narrows the model back: all three axes fall to near-base levels, revealing a consistent division of labor — CPT re-organizes and lifts the shared capability substrate; SFT cashes it out onto target tasks. We argue this reframes biological sequence not as a competitor for a foundation model’s capacity but as a form of structured scientific data that reshapes its capability profile, and that CPT and SFT should be budgeted as complementary rather than substitutable stages. All checkpoints, evaluation code, and per-example outputs are public. Highlights A training-free re-analysis of one 26B MoE lineage isolates the effect of biological continued pretraining (CPT) from tokenizer changes and from fine-tuning. Biological CPT does not cause catastrophic forgetting; it raises general knowledge (MMLU +13 pts) and code generation (MBPP pass@1 0.33 → 0.63). CPT also makes chain-of-thought reasoning 41% shorter and near-backtrack-free while pre-serving accuracy — an effect invisible to accuracy metrics. A consistent CPT-lifts / SFT-narrows division of labor recurs across four axes, reframing biological sequence as structured scientific data that reshapes a model’s capability profile. The Bigger Picture Adapting a general-purpose AI model to a specialized domain — here, the language of DNA and proteins — is usually assumed to come at a cost: teach it biology and it forgets how to reason about everything else. This “no free lunch” intuition shapes how practitioners budget compute and whether they attempt domain adaptation at all. We test the assumption directly, and without running any new training, by comparing three snapshots of the same model taken before and after biological training. The result overturns the intuition: feeding the model raw biological sequence made it better at general knowledge, at writing code, and even changed how it reasons — producing shorter, more decisive chains of thought without losing accuracy. The gains appear during the sequence-pretraining stage and are partly given back during task-specific fine-tuning, revealing that the two stages play complementary rather than interchangeable roles. This suggests a broader principle for data-centric AI: structured scientific data — biological sequence today, and by extension code, mathematics, and chemistry — is not merely knowledge to be absorbed but a lever that reshapes what a foundation model can do.