Question

a farmer sprayed his cornfield with a pesticide and was pleased to note that most of the pest insects had been eliminated. after several years, the pesticide seemed to lose its effectiveness. what is the most likely explanation for this? there was a higher than normal rate of immigration of insects. a few insects possessed genetic traits that prevented them from being killed and these favorable traits were passed to future offspring. the environment changed which altered the chemical composition of the pesticide. new and different types of insects moved into the cornfield.

Explanation:

This is an example of natural selection. When the pesticide was applied, most insects died, but a small number had innate genetic traits that made them resistant. These resistant insects survived, reproduced, and passed their pesticide-resistant traits to their offspring. Over multiple generations, the population of resistant insects became dominant, making the pesticide seem ineffective. The other options do not align with the standard biological explanation for pesticide resistance: insect immigration would not cause a loss of effectiveness over years, the environment does not alter the pesticide's chemical composition in this way, and new insect types moving in would not explain the original pest population becoming resistant.

Answer:

A few insects possessed genetic traits that prevented them from being killed and these favorable traits were passed to future offspring.