Modeling Factors Affecting the Severity of Outbreeding Depression
Suzanne Edmands and Charles C Tmmerman; June 2003
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.