October 2005

Environmentalists raised an outcry when the British Centre for Ecology and Hydrology announced the discovery of what has been termed a “superweed” in July of 2005. The single wild mustard plant achieved superweed status in the minds of some when it proved resistant to a powerful weed killer. Scientists discovered the plant in a field that had been used in trials of genetically-modified (GM) oilseed rape, a group of plants which includes those used to produce canola oil. Environmental groups warn that this discovery augurs serious agricultural and environmental repercussions if genetically-modified oilseed rape were to be grown commercially in the UK.

Where's the evolution?

But why would one wild mustard plant cause so much concern, and what does this all have to do with evolution? The answer relies on genetic engineering, the evolutionary history of crop plants, and the power of natural selection.

In genetic engineering, people modify the genome of an organism, usually by inserting a foreign gene. In the case of GM oilseed rape, the rape plants carry a bacterial gene which makes them resistant to a powerful herbicide. This bacterial gene encodes a protein that chemically alters and detoxifies the herbicide.

Planting GM rape might appeal to farmers (at least in the short term) because they would be able to treat their fields with a weed-killing herbicide without harming their rape plants. However, this practice raises a variety of concerns — one of which is the evolution of superweeds.

Superweeds can be produced when a foreign gene from a GM crop is unintentionally passed to a related weed, which then also expresses the benefit of genetic modification — in this case, resistance to an herbicide. The herbicide then becomes useless against that weed. This transfer of genes from one species to another can occur in plants because different plant species sometimes form hybrids. The closer the evolutionary relationship between two plant species, the more likely they are to hybridize. Wild mustard (the weed) and rape (the crop) are evolutionary cousins. In fact, wild mustard is the evolutionary cousin of many crop plants because farmers developed these crops by artificially selecting the ancestors of wild mustard until they evolved into useful crop plants, such as broccoli and cauliflower.

This transfer of genes from a GM crop to a weed seems to have occurred in the case of the British “superweed” — although it is not yet clear if the plant is fertile. However, scientists and the public are less concerned about the fate of this particular “superweed” than the possibility of future superweeds, in general. After all, if it happens once, it could easily happen in other cases, and once a fertile superweed carrying a gene for herbicide resistance evolves, it is likely to be favored by natural selection in any environment in which farmers use that herbicide. The prevalence of superweeds could encourage farmers to use more toxic herbicides and could eventually escalate into a dangerous arms race — wherein agriculturists develop and use ever-more-powerful herbicides, as weeds, in turn, evolve resistance to them.