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Summary Replicated trait evolution can provide insights into the mechanisms underlying the evolution of biodiversity. One example of replicated evolution is the awn, an organ elaboration in grass inflorescences.Awns are likely homologous to leaf blades. We hypothesized that awns have evolved repeatedly because a conserved leaf blade developmental program is continuously activated and suppressed over the course of evolution, leading to the repeated emergence and loss of awns. To evaluate predictions arising from our hypothesis, we used ancestral state estimations, comparative genetics, anatomy, and morphology to trace awn evolution.We discovered that awned lemmas that evolved independently share similarities in developmental trajectory. In addition, in two species with independently derived awns and differing awn morphologies (Brachypodium distachyonandAlopecurus myosuroides), we found that orthologs of theYABBYtranscription factor geneDROOPING LEAFare required for awn initiation. Our analyses of awn development inBrachypodium distachyon,Alopecurus myosuroides, andHolcus lanatusalso revealed that differences in the relative expansion of awned lemma compartments can explain diversity in awn morphology at maturity.Our results show that developmental conservation can underlie replicated evolution and can potentiate the evolution of morphological diversity.