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Keywords:

  • Bounded Hybrid Superiority model;
  • Dynamic Equilibrium model;
  • endogenous selection;
  • Environmental Gradient model;
  • exogenous selection;
  • genotype–by–environment interaction;
  • intrinsic rate of increase;
  • stable hybrid zone

We have studied reciprocal transplant gardens involving the hybrid zone between basin and mountain big sagebrush (Artemisia tridentata) in Salt Creek Canyon, Utah, for 9 years. Previously, we showed that the parental taxa and hybrids had superior reproductive and vegetative performance in their native garden. These earlier data supported the Bounded Hybrid Superiority model. Now, after 9 years, we find that the mountain seed source plants have greater relative fitness than middle hybrid zone seed source plants in the middle hybrid zone garden. These results may be due to plant density and climatic factors more conducive to mountain seed source growth than that of either basin or middle hybrid zone seed source plants. On the other hand, these fitness estimates do not take into account the timing of reproduction, which together with the age-specific survival rate, can profoundly affect lifetime fitness. The intrinsic rate of increase (r) takes both of these factors into account, providing another estimate of fitness. Middle hybrid zone seed source plants had the greatest rate of increase in both the middle hybrid zone and mountain gardens and a greater rate of increase than either parent in the basin garden. This is most likely due to the greater reproductive performance of middle hybrid zone plants earlier in life than either parental taxon. These results partly support the Bounded Hybrid Superiority model and show the importance of long- term studies of hybrid fitness. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 213–225.