Here's why:
* Natural Variation: Within a mosquito population, there is natural variation. Some mosquitoes may have genes that make them more resistant to a particular pesticide.
* Selective Pressure: When a pesticide is applied, it acts as a strong selective pressure. Mosquitoes with the resistance genes are more likely to survive and reproduce.
* Increased Frequency: Over time, the resistant mosquitoes pass on their genes to their offspring, increasing the frequency of resistance genes in the population.
* Adaptation: Eventually, the majority of the population may be resistant to the pesticide, making it less effective.
This is a classic example of natural selection, where the environment (the pesticide) favors certain traits (resistance) and these traits become more prevalent in the population.