Having established a baseline value of genetic equilibrium in a desert cactus population, it is possible to study the forces at work that bring about evolutionary change.
Peccaries are great predators of cacti, which form one of their favorite foods. When peccaries discover an untouched population of cacti, they go to work and quickly exploit the new source of this delicacy.
With lots of choice, and lots of new material to eat, peccaries naturally choose to consume those cactus plants with the fewest spines. Even with their tough mouths, they prefer to eat the cacti with 70 spines first, before going on to tackle the plants with 80 spines.
In this new, peccary-filled, environment, the cacti with more spines are better adapted, more fit, and get eaten less often.
As a result, at flowering time there are more cacti with higher spine numbers; thus, there are more of their alleles going into pollen, eggs, and seeds for the next generation.
Over long periods of time, in the constant presence of hungry peccaries, the population of cacti will gradually shift in the direction of the more heavily spined cactus varieties. The later (newer) distribution curves will show a virtual absence of low-spine-numbered plants and a much greater preponderance of high-spine-numbered plants.
This is called directional selection.
Directional selection favors one extreme value for a particular trait in a distribution of these values.