Structural Underdetermination in Evolutionary Explanation_ Why Selection-First Accounts Cannot Converge to Causal Proof

Abstract This paper presents a structural audit of selection-first evolutionary explanations, understood as accounts of the form “Trait X exists because it was selected for.” The analysis does not dispute evolution, common descent, or natural selection as real biological processes, nor does it propose alternative mechanisms. Instead, it examines whether selection-first explanations, as commonly employed, admit convergence toward causal determination in principle. Using general criteria for causal explanation—mechanistic generation, eliminability of alternatives, and contraction of hypothesis space under accumulating evidence—the paper shows that selection-first explanations systematically fail to converge when used as token-level causal accounts of realized trait form. Selection operates as a filter on existing variation rather than a generator of constrained causal pathways. As a result, multiple inequivalent genetic, developmental, and selective histories remain compatible with the same observed outcome, even under refinement. The paper further demonstrates that post-hoc fitness narratives, non-injective evidence-to-cause mappings, and the absence of eliminative failure modes prevent explanatory contraction. By contrast, population genetics, molecular biology, developmental biology, and constrained phylogenetics are shown to converge because they impose explicit mechanisms and eliminable structure. The conclusion is not that evolutionary biology is false or incomplete, but that selection-first explanations must be reclassified. When offered without independent generative constraints, they function as heuristic or narrative frameworks rather than terminating causal explanations. This result follows from structural considerations alone.