Vavilovian mimicry occurs in plants where a weed comes to look like a crop plant. It is named after Nikolai Vavilov, a Russian plant breeder who discovered the idea. Before herbicides, weeds were plucked by hand. This has been done for thousands of years. The weeds come to look like the crop because the weeders pick those weeds which look most different. Vavilovian mimicry is caused by unintentional selection by humans. The weeds that survive pass on their genes. Gradually, all the weeds look more like the crop plant.