Two properties of a gene-for-gene coevolution system under human perturbations

Recently, the gene-for-gene host-parasite coevolution model of Leonard was extended by incorporating two kinds of perturbations. The first kind was th e natural perturbations that include those caused by pathogen migration bet ween the two subpopulations of the host. forward and backward mutations in the host or pathogen populations, and some others. The second kind was huma n perturbations, such as constantly increasing the percentage of the resist ant genotype within the host population each season. In this study, we quan titatively compared the two kinds of perturbations and extended the constan tly changing human perturbation to include non-constant perturbations that are more likely to occur in the real world. Two properties of the modified Leonard model were revealed from this study. First, when both human perturb ations and natural perturbations are involved, the effects of natural pertu rbations are very small compared with those of human perturbations. This fi nding ensures that, in the study of human perturbations, we can simplify th e study by ignoring the effects of natural perturbations. Second, through t he simulation of nonconstant perturbations, which assumes that the proporti on of the resistant genotype of the host population increases over time. we found that the model reproduces the "boom and bust" epidemic cycles that a re often found in agroecosystems.