A population of spiny cacti is in genetic equilibrium, with no forces of selection acting on it, the distribution curve of number of cacti showing a particular number of spines is broad and symmetrical.
A road is built quite close to the study site, which keeps away the peccaries and the parasitic insects, but with the road comes the tourists.
In many desert areas of the United States, passing cactus aficionados like to pick up a souvenir cactus to take home with them after a day-trip out into the desert. This is a serious problem in some areas because the tourists always take the better looking cacti, and these happen to be the ones with the middle-spine-numbers.
Years of collecting have left their toll on the roadside cacti. In this environment, it is maladaptive to be good looking and have a reasonable number of spines. Low-spine-number plants are not picked because they don't "look right", and high-spine-number varieties are left alone because they are too hard to pick.
Gradually, the gene pool changes in favor of the two extreme spine number types. Disruptive selection works against those phenotypes that fall into the midrange of values.
Disruptive selection favors both extreme values for a particular trait in a distribution curve.