A farmer sprays insecticide on his crops and notices that the insect pest problem disappears. A year later he tries the same insecticide only to find that he did not achieve the same result. The insect population shows resistance to the insecticide. This is an example of ____ acting on a population to change its characteristics.

This is an example of natural selection acting on a population to change its characteristics. To understand why the insect population developed resistance to the insecticide, we need to consider the basic principles of natural selection.

Natural selection is a process where certain traits or characteristics become more or less common in a population over time, based on their ability to survive and reproduce. In this case, the insecticide was initially effective in reducing the insect pest problem. However, some individual insects may have had genetic variations that made them resistant to the insecticide, perhaps due to a mutation or pre-existing genetic diversity within the population.

When the farmer sprayed the insecticide, most of the insects susceptible to it were killed, but the few resistant ones survived and passed on their resistant traits to their offspring. Over time, the resistant insects became more prevalent in the population, making the insecticide less effective.

This process of natural selection occurs when certain individuals with advantageous traits survive and reproduce more successfully than others. In the case of insecticide resistance, the resistance trait allowed some insects to survive and reproduce, while the susceptible ones were removed from the population. This change in the population's characteristics is a result of natural selection acting on it.