Heterozygote Advantage in Theory
When two populations of any sexual organism are separated and kept isolated from each other, the frequencies of deleterious mutations in the two populations will differ over time, by genetic drift. It is highly unlikely, however, that the same deleterious mutations will be prevalent in both populations after a long period of separation. Since loss-of-function mutations tend to be recessive (given that dominant mutations of this type generally prevent the organism from reproducing and thereby passing the gene on to the next generation), the result of any cross between the two populations will be fitter than the parent.
This article deals with the specific case of fitness overdominance, where the fitness advantage of the cross is caused by being heterozygous at one specific locus alone.
Read more about this topic: Heterozygote Advantage
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