I believe that most modern biologists agree that "species" is an out-dated idea; still useful in some ways, the way Newtonian Physics is useful when launching satellites/spacecraft, even though it is known to be wrong (Einsteinian Physics is much closer to correct; but for most purposes the difference is negligible).
My education on the subject is a good while back, and one's education is usually a decade or two out of date at the time it is received, so my education on the issue is probably cutting edge for the 80's or so. I have read recently that the field of evolution has been at a stand still for a "generation," a vague term that, based on the scientists mentioned, is likely to be at least two generations, so maybe my education was cutting edge since there wasn't a cutting edge at the time.
It was always very evident to me that the theory of evolution I was being taught wasn't fully baked as speciation was kind of an important part of the theory, and the definition of species resulted in humans being, necessarily, supernatural beings. But the scientific community was preaching evolution as inarguable fact. All that sort of thing does is damage credibility.
I tend to think that Molly carried the gene and that her siblings didn't or that her siblings do and that if exposed to magic for any long span of time that they would come into magic themselves.
Plenty of genes are "activated" by things that happen
in utero. Based on WoJ, I think there is a strong possibility that magical talent may be one of them.
As mentioned before, I believe there is an obscure WoJ that most wizards are descendants of Changelings.
I remember plenty of discussion about that idea here, but I don't recall the WoJ. If anyone can find it, that would be appreciated.