Excess use of herbicides, pesticides, etc., has only resulted in the selection of resistant varieties in a much lesser time scale. This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell. Hence, resistant organisms/cells are appearing on a time scale of months or years and not centuries. These are examples of evolution by anthropogenic action. This also tells us that evolution is not a directed process in the sense of determinism. It is a stochastic process based on chance events in nature and chance mutation in the organisms.
In a mixed population, those that can better-adapt, survive and increase in population size. Remember that no variant is completely wiped out.