The argument for science fiction, granted social science rather than hard science: the effects of genetic drift on a mixed population with two distinct genomes. In the "old days", I might have used the word "races" to discuss the genetic combining of elves and humans, but that's not kosher anymore. What happens is that traits and skills get mixed in a population: 1/4 genetically the same in one population, 1/2 mixed genetically, and 1/4 genetically the same in the other population. Half the population would have no magic, and the other half would have an over-abundance in basic Mendelian terms. Of course, that's for just one gene. The possible manifestations increase with the number of genes.
The problems of mixing gets more complicated because genes don't stay on their original chromosomes. They jump [transfer] from maternal to paternal chromosomes and back again. So, you never quite know which traits might appear in a physical individual. "Dark Solstice" is set in a background where most of the population has mixed genetically ... while still being governed by the original mostly elvish population four hundred years later. --- Try to fit that into a query or snynopsis.