20.1  Acquiring resistance to herbicides

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Weedy plant species can be present in very high numbers and exhibit characteristics such as high fecundity, plasticity, resistance to environmental stress and considerable genotypic diversity. Weeds of agriculture are often exposed to catastrophic environmental changes when attempts are made to eradicate them. Total eradication is rarely successful because weeds possess genetic traits that enable them to survive these stresses. In general, genetic variation is essential if a population is to survive catastrophic environmental change. The source of this variability is assumed to be the underlying mutation rate at the gene level. So, herbicide application is a powerful selection pressure applied to huge populations, exposing any individuals with a genetic ability to survive herbicide treatment. While the population as a whole suffers high mortality, the herbicide is effectively selecting for rare individuals that possess any genetically endowed traits (resistance genes) that enable them to survive at the herbicide dosage used. These survivors produce seed and contribute to the gene pool of subsequent generations, enriching the population with resistance genes. Thus, herbicide resistance results from selection pressure working on genetic diversity in a classical Darwinian sense.