A mutation occurs in a pest population.
If this mutation is positive, that is, useful, and ensures the resistance of an individual to a new pesticide, then this mutation is fixed by natural selection, that is, individuals resistant to environmental conditions mainly survive, and individuals who do not have a mutant gene die.
As a result, this mutation spreads throughout the entire population (insects reproduce very quickly), and almost every individual has a mutant gene that ensures its resistance to toxic chemicals.
Thus, under the influence of natural selection, a population is formed that is resistant to this toxic chemical.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.