The introduction of the herbivores would disrupt the genetic equilibrium in the population because the herbivores would act as a mechanism of natural selection.
The presence of anthocyanin in the stem is determined by the presence of the dominant allele ANL, so the genotypes of the plants that have anthocyanin are ANL/ANL and ANL/anl. The plants that do not have anthocyanin in their stems are homozygous for the anl allele and the herbivores would eat these plants more often, reducing the frequency of the anl allele in the population.
