A coevolutionary model was developed to test interactions between diploid plants and herbivores using genetic algorithms on a spatial lattice. Simulated plants carried defensive genes and herbivores carried genes coding for resistance (metabolism of herbivore defense) in gene-for-gene synchrony. Collectively these genes are referred to as defensive/resistance genes (DR-genes). Genes were linked on chromosomes. Regulatory genes modified both dominance at these DR loci and the tradeoff cost involved in producing either defense or resistance. We tested the effects of varying a) the number of DR-loci, b) the ratio of the number of herbivore:plant generations, c) the shape (square vs. long and thin) and function (torus vs. island) of the lattice, and d) herbivore encounter rate on plant progeny dispersal distance.

Increasing both the number of DR-genes and the ratio of herbivore:plant generations caused a tighter coevolutionary response between plants and herbivores. Plant defense was highly sensitive to herbivory but not to increasing encounter rates. Plant DR-genes were selectively disadvantageous with only one lucus but selectively favored with two or more loci. Increasing the number of herbivore:plant generations caused increased fluctuations in herbivore resistance gene frequencies and a decrease in the lag time in herbivore response to changes in plant defensive gene frequencies. The relationship between heterotroph and autotroph DR-genes increased exponentially with increasing numbers of DR-loci. This relationship suggests that autotrophs benefit from increased diversity of defense that causes a relative increase in cost for the heterotrophs. The shape of the lattice interacted with lattice function, resulting in high species persistence on wraparound habitats and the greatest extinction likelihood on rectangular islands. Low to moderate herbivore encounter rates increased plant progeny dispersal distance while high herbivore encounter rates tended to reduce dispersal distance. The frequencies of genes coding for plant defense and herbivore resistance were dynamic for thousands of generations, despite the homogeneous lattice. This interaction may increase extinction probabilities in fragmented habitats.

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