Figure 28. Diagrammatic representations of two out of four patterns of gene frequency changes in Leonard’s hypothetical host-parasite models. A. Stable equilibrium: frequencies of the resistance and virulence genes, plotted in a phase plane, spiral inward toward a stable, internal equilibrium point (EP). B. Unstable equilibrium: gene frequencies spiral outward away from an unstable equilibrium point (EP) until virulence becomes fixed in the parasite population and resistance is lost in the host population. 

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