Limits to the evolution of herbicide escape and tolerance in the agricultural weed <i>Amaranthus palmeri</i>

In response to novel anthropogenic stresses, defense strategies, including resistance, tolerance, and escape, can evolve. However, if the evolution of one or more of these strategies is limited by weak natural selection or a lack of genetic variation, then a mixed strategy (e.g., resistance and tolerance) is unlikely to evolve. To determine the mechanisms that limit the evolution of defense strategies, we studied escape from and tolerance to glyphosate herbicide in Amaranthus palmeri, an agricultural weed that has evolved glyphosate resistance. We grew A. palmeri in fields planted with corn, soybean, or no crop; manipulated their exposure to glyphosate; and measured escape and tolerance. We did not detect selection or genetic variation for glyphosate escape in any agricultural environment, suggesting that a mixed strategy of resistance and escape is unlikely to evolve in A. palmeri. We also did not detect selection for glyphosate tolerance, but there was genetic variation for tolerance in a corn crop environment, suggesting the potential for a mixed strategy of resistance and tolerance to evolve in A. palmeri in only a subset of environments. These results suggest that exposure to herbicides is unlikely to cause the widespread evolution of a mixed defense strategy in agricultural weeds.