Answer:
The phenomenon of head lice becoming resistant to the substances used to treat them is an example of natural selection in action.
Initially, when a new substance is used to treat head lice infestations, the majority of the lice population may be susceptible to it. However, some individual lice may have a genetic variation that makes them resistant to the substance. These lice survive and reproduce, passing on their resistant genes to their offspring.
Over time, as the susceptible lice die off and the resistant lice continue to reproduce, the overall lice population becomes more resistant to the substance. This is because the resistant genes are now more common in the population.
This process is known as natural selection. In this case, the substance used to treat head lice infestations is acting as a selective pressure, favoring the survival and reproduction of lice with the resistant genes. As a result, the resistant genes become more common in the population over time.
This is why, in the study mentioned, 82% of head lice were found to be resistant to the substances used to treat them. The resistant genes have become prevalent in the head lice population due to the selective pressure of the treatments used.