The model shows that qualities are most easily optimised in small partially isolated populations. Such populations are common in many species including humans. Extreme human mobility is very recent. Before 1900 the author's own name occurred in only two small areas. In such circumstances gene flow would be low with most matings being between individuals in the same or closely adjacent populations.
Crosses sometimes show heterosis but any apparent superiority is deceptive.
In the second generation genes from the two sources recombine in complex ways. Fewer will survive to reproduce. If avoidance of crossing is due to genetic factors such genes will become more common because more progeny are surviving from the more closely related matings. Initial restrictions may be as simple as different courtship signals or breeding periods. Quail, for example, have been shown to prefer the theoretical optimum of cousins as mates' Quail, for example, have been shown to prefer the theoretical optimum of cousins as mates rather than those more or less closely related.