The farmer's shadow
Around 10,000 BCE in the Fertile Crescent of the Middle East, the first farmers began cultivating wild grasses. This act of deliberate planting created a new, human-made environment and triggered an evolutionary arms race. The farmers selected for desirable traits in their crops, like wheat (Triticum) and barley (Hordeum vulgare), but they also applied an accidental, or "unconscious," selection pressure on the weeds growing in their fields. This process of a weed evolving to resemble a crop is now called Vavilovian mimicry.
Two weeds in particular became master impersonators: wild rye (Secale) and wild oats (Avena). Early farmers would weed their fields by hand and save seeds for the next season. Any weed that looked too different from the crop was pulled. Any weed seed that was a different size or weight from the crop seed was separated during winnowing. Over many generations, this relentless, albeit unintentional, selection favored individual rye and oat plants that most closely mimicked the wheat and barley. They evolved larger seeds, a more upright growth habit, and a synchronous ripening time. They also evolved a non-shattering rachis—the tough stem that holds the seeds to the plant, a trait of all domesticated grains. This prevented their seeds from scattering before the harvest, ensuring they would be gathered, stored, and replanted right along with the intended crop.
From pest to protagonist
The mimicry was so successful that rye and oats became permanent, unavoidable contaminants in wheat and barley fields. The farmers were duped, accidentally cultivating their own pests. But the story took a turn as agriculture spread from the temperate Fertile Crescent north into the harsher climates of Europe. The pampered wheat and barley crops often struggled in the colder temperatures and poorer soils. The mimic, however, thrived. Rye and oats, being inherently hardier, often produced a better yield than the crop they were impersonating. Farmers in these new regions began to favor the weeds, and these "secondary crops" were born. Archaeological evidence shows rye being cultivated as a crop in its own right by the Bronze Age, around 4,000 years ago.
The botanist who identified this phenomenon was Nikolai Ivanovich Vavilov (1887-1943). He developed the theory of "centers of origin," identifying the geographic areas where domesticated plants first arose and showed the highest genetic diversity. His work it put him in direct opposition to the politically powerful Trofim Lysenko, whose anti-Mendelian ideas were favored by Joseph Stalin. In August 1940, Vavilov was arrested on false charges of spying while on a seed-collecting expedition in Ukraine. He was sentenced to death, a sentence later commuted to 20 years' imprisonment. On January 26, 1943, the man who devoted his life to ending famine died of starvation in a Soviet prison.