POPULATION-GENETICS AND THE EVOLUTION OF HERBICIDE RESISTANCE IN WEEDS

Numerous factors, including mutation, selection, inheritance, mating s ystem, and gene flow are important in the evolution of herbicide resis tance in weeds. Spontaneous gene mutation is believed to be the main s ource of genetic variation for resistance evolution in a geographic re gion in which resistance has not been detected previously. Despite mut ation frequencies that are probably very low, the probability of occur rence of at least a single resistant mutant in a susceptible populatio n may be high for weed species with high fecundities and large populat ion sizes. Subsequent repeated treatments with herbicides having the s ame mode of action could lead to the rapid evolution of predominantly resistant populations. Rare dominantly inherited resistance mutations spread significantly more rapidly than recessive mutations in random m ating populations, but at roughly the same rate in highly self-fertili zing species. Gene flow, through the movement of pollen or seed from r esistant weed populations, may provide a source of resistance alleles to adjacent or nearby susceptible fields. Mathematical models indicate that the strength of selection imposed by a herbicide and the initial frequency of the resistant phenotype most strong ly influence the rat e of resistance evolution. The models predict that the most effective strategies to manage resistance are to reduce the intensity of selecti on by herbicide and to limit the migration of herbicide-resistant seed .