Our brains continue developing after birth.  That development is driven by experience.  If, during the post-birth wiring period we are exposed to certain things, we will learn those things very, very well...or conversely.

I completely agree with this.  some of my earliest memories of my mother are of her telling me, over and over, that I could do anything I wanted.  that I could be anyone I wanted.  she said it so much I started believing it.  it has served me so well in life.  it is not very common for someone of my background to have the life I have had and I totally believe I owe it to her and her drilling self confidence into me to a fault.  I say to a fault because "fault" is how some people see what they would call my over confidence.
and I dont necessarily disagree with them but on the whole it has shaped my life in very positive ways.

As Kramer once said, "Mother nature is a mad scientist". In other words, we don't know.

My guess is that it's a combination of genetics, upbringing, enviornment, and everything else that leads to this.

You can park a couple siblings from the same household in front of the same musical instrument, give them the same lessons with the same teacher and the one might get it and the other not, and there is no telling which one, nor why.  And then have the grotesque disappointment of watching the one who didn't get it, turn into a trial lawyer.

  Are the difference magnified because, not surprisingly, once we got old enough to have some choice with our parents I spent more time on sports and to a lesser extent school while he spent more time on music and painting? I'd guess so, but it would seem unwise to ignore how our innate abilities influences our choices about what to practice.

  Do we understand how all the variables influence development. Of course not, but suggesting that because we don't understand how and why nature differentiates doesn't mean we aren't all born different with different capacities for developing different thngs.

However, there are different issues here: being a genius doesn't necessarily mean that you'll go far, and not being a genius doesn't mean that you can't go far.

Going far means having the essential combination of two characteristics: talent and discipline.

The question of a "hard work quotient" would be fairly easy to manufacture, a "HWQ." Some people we would find are simply more hard working than others, they would have a higher "HWQ." And then we could formulate a new bell curve on that basis (it is the seductive magic of the bell curve that it will work on virtually any attribute). The notion of "hard work" is tied into our notion of free will in the sense that we think we could always just "work harder" if we just wanted to. But the question of wanting to is precisely what would be at issue. It would come down to a question analogous to Plato's Meno: How do we teach virtue? In this case the virtue of working hard. In the end it would be all to easy to wind up with the inherently hard working vs the inherently lazy instead of the inherently intelligent vs the inherently stupid.

None of this is necessary of course, I'm merely pointing out that genetic meritocracy is flexible and does not require one kind of talent be highlighted over another.

Finally I would note that even accepting the schema presented in the OP (which I think def has a ring of truth to it) there are certain kinds of genius that seem to me unreducible to practice. Many brilliant mathemetician do their greatest work very eary in life. Is an Einstein reducible to mere practice of Relativity theory? Can the literary fecundity and historical uniqueness of Kierkegaard be reduced to many hours of "practicing existentialist philosophy?"

She expects to be able to do it and has spent the required number of hours practicing, so she does do it.

Local HS, Christian Oaks, wins state championships in almost every sport they compete in almost every year, in large part because they expect to.

Last year's quarterback roster? Joe Montana's kid, Wayne Gretzky's kid and Will Smith's kid.

Why?

Because they USE them for non-trivial tasks, like play and other personal interaction.

That's the key to ALL learning, actually: small, diverse groups engaged in meaningful tasks in which ALL are regarded as having certain, but unspecified, competence, interactively re-inforcinig and instructing one another....

When he as finished these deliberations, he hears, in his mind's ear, in the flash of a second, the piece played in its entirety.  From then on the orchestral score is inscribed in his brain, and can be written down at will, even among distractions.

The subject is fascinating, and so many of the comments are profoundly interesting; some even epiphanous. And therein lays my complaint.

So many personal stories, observations, reflections, and opinions; so little time to properly absorb, digest, and appreciate their meaning(s). Each one, at least to me, deserves its own blog, or volume, or something.

It's like a Picasso exhibit at the MOMA. Having studied him just a tiny bit (just enough to be smitten) I took a date there a few years ago. Upon arriving, I looked at the first drawing that caught my eye; and I looked, and looked, and looked.......... Finally, Sarah poked me in the ribs, and with an impish smile said, "there's a few more pictures here you might want to see, Shooter." I could have looked at that drawing for hours, and seen something new, or different, each minute of those hours.