• Coevolution;
  • herbivory;
  • intraorganismal selection;
  • mosaicism;
  • plant defense

We model direct fitness benefits of genetic mosaicism for a long-lived tree in coevolution with a short-lived herbivore to test four hypotheses: that mosaicism reduces selection on the herbivore for resistance to plant defenses; that module-level selection allows the individual tree to adapt to its herbivore; and that this benefits the tree population, increasing average tree fitness and reducing local adaptation of the herbivore. We show that: mosaicism does not sufficiently reduce selection for resistance in the herbivore to benefit the tree; that individual trees do benefit from module-level selection when somatic mutation introduces new defenses; and that mosaicism does reduce local adaptation in the herbivore, which increases average tree fitness. These results are robust to varying genetic assumptions of dominance and the somatic mutation rate, but only hold for sufficiently long-lived trees with relatively strong selection. We also show that a mixed reproductive strategy of primarily asexual reproduction interspersed with occasional sexual reproduction is effective in coevolving with the herbivore, as it maintains beneficial allele combinations. Finally, we argue that intraorganismal genetic heterogeneity need not threaten the integrity of the individual and may be adaptive when selection acts concordantly between levels.