Some Garfinas have thorns and some do not. You collect data from wild Garfinas populations and discover that plants with thorns have significantly less damage from herbivores (predators) then plants without thorns. Herbivore damage may require plants to put energy into repairing themselves that otherwise could go towards reproduction. Herbivores can also damage the reproductive structures of the plant or kill the plant.
You decide to use seed production as a measurement of reproductive success. By counting the number of seeds and comparing it to the amount of herbivore damage you show that, as predicted, larger amounts of herbivore damage equal lower reproductive success, and smaller amounts of herbivore damage equals higher reproductive success.
What is occurring for the thorn phenotype in Garfinas plants?