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The predictability of evolution is debatable, with recent evidence suggesting that outcomes may be constrained by gene interaction networks [1]. Whole-genome duplication (WGD; polyploidization—ubiquitous in plant evolution [2]) provides the opportunity to evaluate the predictability of genome reduction, a pervasive feature of evolution [3, 4]. Repeated patterns of genome reduction appear to have occurred via duplicated gene (homeolog) loss in divergent species following ancient WGD [5–9], with evidence for preferential retention of duplicates in certain gene classes [8–10]. The speed at which these patterns arise is unknown. We examined presence/absence of 70 homeologous loci in 59 Tragopogon miscellus plants from five natural populations of independent origin; this allotetraploid arose ∼80 years ago via hybridization between diploid parents and WGD [11]. Genes were repeatedly retained or lost in …