It's important to keep in mind that selective pressure dictates how fast a beneficial mutation spreads in the population. If the terminte parasites were very rare, and there are only ever one or two per termite colony that eat a very small percentage of the food, any termite queen wo evolves the ability to recognize the parasite just a bit better would not have much of advantage compared to the other colonies. The positive mutation may be lost by chance again. However if the parasites were everywhere and take 50% of a colonies food, the small improvement in detection would lead to a significant advantage in reproduction of that termite colony, and thus the mutation would spread faster.
