Modeling factors affecting the severity of outbreeding depression

Hybridization between populations may cause either increased fitness ("hybrid vigor") or decreased fitness ("outbreeding depression"). Translocation between populations may therefore in some cases be a successful means of combating genetic erosion and preserving evolutionary potential, whereas in other cases it may make the situation worse by inducing outbreeding depression. Because genetic distance alone is a poor predictor of the success or failure of hybridization, we developed a computer model (ELAB) to explore other factors affecting the consequences of hybridization. Our model simulates diploid, unisexual populations following Mendelian rules, and in this study we used it to test the effect of a variety of parameters on both the magnitude and duration of outbreeding depression. We focused our simulations on the effects of (1) divergence between populations, (2) the genetic basis of outbreeding depression (disruption of local adaptation vs. intrinsic coadaptation), (3) population parameters such as mutation rate and recombination rate, and (4) alternative management schemes (50:50 mixture vs. one migrant per generation). The magnitude of outbreeding depression increased linearly with genetic distance, whereas the duration of outbreeding depression showed a more complex curvilinear relationship. With genetic distance held constant, magnitude increased with larger population size, lower mutation rate, cross-fertilization, and higher recombination rate, whereas duration increased with larger population size and partial self-fertilization. Fitness problems caused by disruption of local adaptation were stronger but more transient than those caused by a disruption of intrinsic coadaptation. Finally, simulations showed that, depending on the genetic basis of outcrossing problems, recurrent transfer of only one migrant per generation into a population of 100 individuals could cause as much or more damage as a one-time 50:50 mixture.