1. It is widely assumed that organisms which evolve resistance to a triazine suffer from a fitness deficit in the absence of that herbicide. Arguments for this view are examined, and the published evidence is discussed. \ud \ud 2. A simple model is developed to examine the genetics of resistance in triazine-resistant plants, based on the assumption that there is a founder effect operating. The model examines the hypothesis that the extent to which plants in the sprayed population are related will increase rapidly under continual selection, even when there is a significant input of genes from non-selected populations. \ud \ud 3. The possible consequences of the above hypothesis on the validity of competition experiments are discussed.\u
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