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Crop domestication is an adaptive process that transforms a wild plant into a domesticated species that can reared and maintained for human use. Though there are hundreds of thousands of flowering plant species, only a small fraction has ever been domesticated. Successful domestication is likely influenced by a number of key plant characteristics, including its life history, the usefulness of a crop for early societies, and the maintenance of a large effective population size. Although many studies have sought to identify individual loci with large effects on domestication traits, we argue that relevant phenotypes are likely controlled by a large number of loci, most of relatively small effect. Most of these alleles were probably selected from standing genetic variation present in the wild ancestor rather than new mutations. Both archaeological evidence and quantitative genetics suggest that the process of domestication was in most cases gradual, likely lasting several millennia. We end by discussing how these findings from the past may inform future efforts to domesticate new species.