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In plant biology, Vavilovian mimicry (also crop mimicry or weed mimicry[1][a]) is a form of mimicry in plants where a weed evolves to share one or more characteristics with a domesticated plant through generations of artificial selection.[2] It is named after Nikolai Vavilov, a prominent Russian plant geneticist.[2] Selection against the weed may occur by killing a young or adult weed, separating its seeds from those of the crop (winnowing), or both. This has been done manually since Neolithic times, and in more recent years by agricultural machinery.

Vavilovian mimicry is a good illustration of unintentional selection by humans. Although the human selective agents might be conscious of their impact on the local weed gene pool, such effects go against the goals of those growing crops. Weeders do not want to select for weeds that are increasingly similar to the cultivated plant, yet the only other option is to let the weeds grow and compete with crops for sunlight and nutrients. Similar situations include antibiotic resistance and, also in agricultural crops, herbicide resistance. Having acquired many desirable qualities by being subjected to similar selective pressures, Vavilovian mimics may eventually be domesticated themselves. Vavilov called these weeds-become-crops secondary crops.

Another example is rye (Secale cereale), a grass which is derived from wild rye (Secale montanum), a widely distributed Mediterranean species. Rye was originally just a weed growing with wheat and barley, but came under similar selective pressures to the crops. Like wheat, it came to have larger seeds and more rigid spindles to which the seeds are attached. However, wheat is an annual plant, while wild rye is a perennial. At the end of each growing season wheat produces seeds, while wild rye does not and is thus destroyed as the post-harvest soil is tilled. However, there are occasional mutants that do set seed. These have been protected from destruction, and rye has thus evolved to become an annual plant.[5]

Rye is a hardier plant than wheat, surviving in harsher conditions. Having become preadapted as a crop through wheat mimicry, rye was then positioned to become a cultivated plant in areas where soil and climatic conditions favored its production, such as mountainous terrain.[4]

This fate is shared by oats (Avena sativa and Avena byzantina), which also tolerate poorer conditions, and like rye, grow as a weed alongside wheat and barley. Derived from a wild species (Avena sterilis), it has thus come to be a crop in its own right. Once again paralleling wheat, rye and other cereals, oats have developed tough spindles which prevent seeds from easily dropping off, and other characteristics which also help in natural dispersal have become vestigial, including the awns which allow them to self bury.[4]