Mechanism of outbreeding depression:
In the excellent review article on inbreeding depression and outbreeding depression in animals (Suzanne Edmands, Between a Rock and a Hard Place, Molecular Ecology, vol. 16, no. 3 page 463 published online November 15, 2006), consideration is given to what mechanism might possibly produce such an effect. Mentioned are intrinsic interactions between genes and extrinsic interactions between genes and the environment.
There is something called “overdominance” where a heterozygous state, in which the individual with copies of two different alleles (forms of a gene) is more fit than an individual with two copies of either of the alleles. There is the reciprocal called “underdominance” where the reverse is true. There is something called epistasis, where genes interact with each other and there are effects involving the sex chromosomes. Sometimes the reduction in fitness does not occur until a generation or generations after the first crossbreeding. The article goes into these various possibilities briefly.
The model I have offered does not include a representation of sex chromosomes, so any effect there is not reflected. As for the others, which is right? Or at least which is most important.
In fact, they reduce to the same thing in the model. If two genes interact or if they produce proteins that interact or if they produce effects that are not exposed until the organism interacts with the environment, the arithmetic is about the same.
Thus there are potential mechanisms for the effect we are looking at. The model does not distinguish among them, but that is not our primary interest. The fact is that mixing genes can destroy fertility, whether by effects on the reproductive system or more indirect effects that reduce an organism’s ability ultimately to attempt to reproduce. The big picture remains the same, and the data indicate that the effects are of great concern.
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