Once upon a time in the make believe world of Corporationia, researchers at
land grant universities discovered that they could use pesticides to kill squash bugs and cucumber beetles. So they started spaying the heck out of the plants and they were saved. Yay for science!!! So generation after generation the plant breeders used poisons to protect the squash from bugs. Generation after generation the genetic knowledge of how to deal with bugs got lost as plants without resistance to bugs gained prominence among the plant population. Then the chemical corporations bought up the seed companies, and they discovered that they could
sell more poisons if they sold only those squash seeds that are highly susceptible to squash bugs and cucumber beetles. So the world was saturated with squash containing little genetic memory about how to deal with bugs. And all was well because the corporations sold more poisons, which required them to hire more employees, which bought more things, and the economy grew and prospered.
However, there was a group of country bumpkins which called themselves The Purists. They believed that one of the first rules of healthy living is that they shouldn't poison themselves. So when they planted The Corporation's seeds, the squash failed due to lack of crop protection chemicals. But they didn't all fail. One variety in 20 or 50, or one plant in 100 of some varieties, or 1 plant in 10 of other varieties, still carried some residual memory of how to deal with bugs. The Purists saved seeds from the few plants that survived. They replanted the most resistant. Generation after generation, the genes that provided resistance to bugs got recombined, and the squash grown by The Purists became progressively more resistant to squash bugs and cucumber beetles.