"Looking for Ways to Beat the Weeds" is a recent article in the NYT by Carl Zimmer that looks at the way agricultural plants can evolve weediness.
Scientists have documented three different ways that plants evolve into weeds. Many species, such as barnyardgrass, evolved from wild ancestors. Biologists have found that certain traits make it easier for wild species to become weeds. They already grow fast, for example, and make lots of seeds.Parasitic plants are especially well-suited to the weedy life. They wrap around other plants and send their roots into their hosts’ tissues. Rather than making their own food, parasitic plants steal nutrients from their hosts. The parasitic weeds that invade farm fields have not evolved major differences from the ones that attack wild plants.In other cases, weeds evolved from the union of wild plants and crops. In the 1970s, for example, wild beets in Europe released pollen that fertilized sugar beets growing on farms miles away.Crops can even turn into weeds. “We domesticated a plant from the wild, and somehow it de-domesticated itself — which I think is pretty exciting,” Dr. Caicedo said.Among these crops gone wild is a weed known as red rice. A key step in the domestication of rice was breeding plants that held onto their seeds when farmers harvested them. Red rice evolved fragile seeds that broke off and fell to the ground.The name red rice comes from the russet tinge that the plant evolves as it becomes a weed. Dr. Caicedo and her colleagues suspect the color is produced from a pigment that helps the seeds go dormant — a trait that’s good for a weed but bad for a crop.“If you’re a farmer, you want your seed to start growing when you plant it,” Dr. Caicedo explained. When weeds produce seeds, on the other hand, some sprout quickly while others go into suspended animation. Those dormant weeds create a seed bank that can sprout later, when conditions may be better for them. “It’s a fantastic trait for a weed to have. You’re hedging your bets,” Dr. Caicedo said.
These de-domesticated weeds don’t simply go back in time to regain the same DNA as their wild ancestors, scientists are finding.
Once plants become weeds, they keep evolving. New mutations allow some of them to have more offspring than others. Foxtail, for example, evolved to crawl along the ground, where it wouldn’t be destroyed by combine blades.