The desert population of spiny cacti are under attack. Peccaries are eating those plants with low-spine-number causing their alleles to vanish from the gene pool and enriching the remaining gene pool in those alleles that create cacti with high-spine-numbers.
Just as the community of cacti and peccaries are adjusting to one another, a second predator, a parasitic insect, arrives in the study area.
This insect lays its eggs at the base of the cacti's spines. When the grubs hatch, they bore into the cacti to eat the soft inner pulp, to grow, to pupate, and to emerge as new adults later in the year.
Preferring densely spined cacti, these egg-laying parasites more often destroy varieties of plants with larger numbers of spines. An infested cactus rarely survives.
In this new situation, with the cactus population under attack from a predator and a parasite, both extremes of spine value are being removed.
Peccaries are consuming the low-spine-number plants, and the insects are killing the high-spine-number plants. As these gene combinations are removed from the cactus gene pool, there is less and less variety possible in subsequent generations.
The distribution curve plotted year after year becomes sharper and narrower, "tightening up" around a limited spine number that just survives both types of killers.
This is called stabilizing selection even though diversity is decreased and such populations are actually very fragile. These plants can be easily eliminated, and sent into extinction, if the environment changes once more. Further evolution away from the most common variety is unlikely.
Stabilizing selection favors those values of a particular trait that are intermediate between the extremes in a distribution curve.