Many gardeners use a chemical weed-killer to control plant pests in their gardens. However, many weeds have begun to develop a resistance to these chemical poisons. this is an example of what in action?

The example that the weeds have begun to develop a resistance to the chemical poisons used as weed killers is evolution. The weeds of the gardens have grown resistance to those chemicals and evolved as stronger weeds and improved on their defensive mechanism to counter those regularly used chemicals.