Mikkelsen and colleagues (1996) have shown that herbicide-tolerant weeds can develop in one to two generations from transgenic oilseed rape (Brassica napus).
Source: Carl F. Jordan (2002). "Genetic Engineering, the Farm Crisis, and World Hunger", BioScience, June 2002, Vol. 52 No. 6, pp.523-529. and Mikkelsen TR, Andersen B, Jorgensen RB. 1996. The risk of transgene spread. Nature 380: 31.