Female mosquitoes need a meal of blood from a person or other animal in order to produce eggs. It has been discovered that mosquitoes have cells on their antennae that can detect the insect repellent known as DEET. The repellent is not harmful to mosquitoes, but when mosquitoes detect DEET, they will not land on the surface where the DEET has been applied. This protects people from being bitten by mosquitoes. Recently, scientists found some mosquitoes that are resistant to DEET because they do not detect its presence. They bred these mosquitoes and eventually produced a population consisting of about 50% DEET- resistant insects. Identify the process most likely responsible for a mosquito initially becoming resistant to DEET.

Mutation followed by natural selection made mosquitos resistant to DEET

Explanation:

Natural selection selects beneficial alleles, which increase their frequency in the population, resulting in adaptation. Aptitude, which is the contribution of each genotype to the next generation, increases too.  

In many cases adaptations, resulting from the natural selection process can be correlated to environmental factors or selective pressures applied by other organisms or habitats.  

Let us remember that, a mutation is a change or alteration in DNI sequences that introduce new variants. Many of these are eliminated, but some of them might succeed and be incorporated into each individual. These mutations are the ones that have been selected by natural selection.

So, in the exposed mosquitos´ example,

Probably some of the mosquitos in the population suffered a mutation that favored them in not detecting repellent DEET. These individuals developed resistance to the chemical and were able to survive and reproduce, enhancing population sizes again. Natural selection benefited the mutation that gave them resistance.

Let us remember that the term resistance refers to an inheritable change in the population sensitivity, reflected through the consecutive failure of the chemical effects, correctly used in order to cause an effect on the insect population.  

Repellents might produce a genetic modification in the insects, leading them to not detect the chemical. Insects evolve with the capability of tolerating the DEET dose that normally is used to repel mosquitos

The excessive use of DEET leads to the fixation of new genes -by natural selection- that result from mutations in the mosquito genetic material, which makes them become even more resistant to the chemical.

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