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Differentiation between ecotypes is usually presumed to be complex and polygenic. Seasonal patterns of life history in salmon are used to categorize them into ecotypes, which are often considered "distinct" animals. Using whole-genome sequencing and tribal fishery sampling of Chinook salmon, we show that a single, small genomic region is nearly perfectly associated with spawning migration timing but not with adiposity or sexual maturity, traits long perceived as central to salmon ecotypes. Distinct migration timing does not prevent interbreeding between ecotypes, which are the result of a simple, ancient polymorphism segregating within a diverse population. Our finding that a complex migratory phenotype results from a single gene region will facilitate conservation and restoration of this iconic fish., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)