Photo by Lotte Grønkjær licensed under CC BY-NC-SA 2.0
Humans have been altering the genomes of plants for a very long time. Nowhere is this more apparent than in the crops we grow. These botanical mutants are pampered beasts compared to their wild congeners. It is easy to see why some traits have been selected over others, whether it be larger leaves or fruit to munch on, smaller seeds to keep them out of our way, or tough rinds to make shipping easier. However, not all of our crops have been consciously bred for our consumption. Just as many weed species are adapting to herbicides today, some species of plant were able to adapt to the more archaic methods of early farming, which allowed them to avoid the ever watchful eye of the farmer.
This concept is known as Vavilovian mimicry (sometimes referred to as crop mimicry) and it is named after the Soviet botanist and geneticist Nikolai Vavilov (who was later imprisoned and starved to death by Stalin because of his firm stance on basic genetic principles). The idea is rather simple. At its core it involves artificial selection, albeit unintentional. A wild plant species finds certain forms of agriculture appealing. It becomes an apparent weed and the farmer begins to deal with it. Perhaps this plant is a close relative and thus looks quite similar to the crop in question. As the farmer weeds out plants that look different from the crop, they may be unintentionally selecting for individual weeds that more closely resemble the crop species. Over enough seasons, only those weeds that look enough like the crop survive and reproduce, sometimes to the point in which the two are almost indistinguishable.
Rye is an interesting example of this idea. Wild rye (Secale montanum) was not intentionally grown for food. It was a weed in the fields of other crops like wheat and barley. Both wheat and barley are annual plants, producing their edible seeds at the end of their first growing season. Wild rye, however, is a perennial and does not produce seed until at least its second season. Therefore, most wild rye plants growing in wheat or barely fields are killed at the end of the season when the field gets tilled. However, there are some mutant rye plants that occasionally pop up and produce seeds in their first year.
It is believed that these mutant annual rye were harvested unintentionally and reseeded season after season. Over time, other traits likely developed to help push rye into the spotlight for these early farmers. Like many wild grasses, wild rye has weak spindles (the part that holds the seed to the plant). In the wild, this allows for efficient seed dispersal. On the farm, this is not a desirable trait as you end up quickly losing the seeds you want to harvest. Again, by accidentally selecting for mutants that also had thicker spindles and thus held on to their seeds, farmers were unintentionally domesticating rye to parallel other cereal crops. It is believed that oats (Avena sterilis) also originated in this manner.
Photo Credit: Lotte Grønkjær (http://bit.ly/1xMEfVw)
Further Reading: [1] [2]