Causal relations among components and activities are intentionally misrepresented in mechanistic explanations found routinely across the life sciences. Since several mechanists explicitly advocate accurately representing factors that make a difference to the outcome, these idealizations conflict with the stated rationale for mechanistic explanation. We argue that these idealizations signal an overlooked feature of reasoning in molecular and cell biology-mechanistic explanations do not occur in

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... solation-and suggest that explanatory practices within the mechanistic tradition share commonalities with model-based approaches prevalent in population biology. * Acknowledgements: Both authors contributed equally to this work. Earlier versions were presented at the 2014 Philosophy of Biology at Madison workshop and the 2014 PSA Biennial Meeting in Chicago. We are grateful to both audiences and to an anonymous reviewer for helpful feedback.