I'm not going to make a scientifict argument about innate intellectual ability - I don't consider myself qualified enough for that.

I can only speak to my own observations. I have taught a lot of people to juggle. Juggling is an interesting discipline because it's pretty distinct from any other activity that most people have participated in in their life. It also has lower risk of "contamination", since people are unlikely to lie if you ask them if they've juggled before. You can also run this experiment with young kids, etc - you will still see very different rates of skill acquisition. I would bet that any other environmental arguments also fall flat - there are no big pro-juggling or anti-juggling cultures out there.

When you get a group of people together and instruct them to juggle, people will pick it up at very different rates. If you have enough of a sample size of observations, you can see extreme variability - some pick it up after 10-15 minutes, others still can't juggle after a year of trying (believe me, I've seen both). I've also observed brothers, both professional jugglers from a young age and both having invested identical amounts of time and passion into the activity (again, have no incentive to lie), progress at very different rates with acquisition of the skill.

From observing hundreds of people learning to juggle, I strongly believe that there is an "innate aptitude for juggling" in every person. Even people without this innate aptitude can learn to juggle and get to a certain level at the skill, beyond which progress will be so slow as to represent a de-facto "ceiling" on their juggling skill.

If you were to make this same argument in intellectual domains (i.e. chess, as an objective example), the argument gets more muddied in that you could make valid arguments that environment has played a role in "predisposing" someone to more rapid skill acquisition. However, I like to think of skill acquisition as more of a y = mx + b equation (not that all skill aquisition is linear). Every person has a certain rate (slope) at which they can aquire a given skill, and a certain intercept (predisposition) towards the acquisition of that skill. This model also works since someone that acquires skill more slowly can expend more time and achieve the same level of competence, but their ultimate ceiling of skill aquisition is lower (since time is finite).

I can make similar arguments about other domains, having spent time with IMO medalists, IOI medalists and other "outliers" in the intellectual domains. The same examples I showed above still apply, but the data is a lot more messy because it's hard in those cases separate the environmental/cultural effects from the genetic effects. That's why I reach for the juggling example as a clearer argument for that.

I'm not claiming that there's no variation in innate ability, only that one island won't be innately better at juggling than the other (certainly one island will be better at juggling, because they will care about juggling more, pass down more juggling, push forward the juggling sciences, &c. But those won't be innate, biological differences).

In other words: if, 1000 years later, you somehow stole an infant from bad-juggling island and had them raised on good-juggling island, they'd likely behave as you'd expect a good-juggle-islander to behave, despite their provenance. (Stipulate that there aren't outwardly evident signs of which island you're from, like inherited skin tone or whatever, which would alter your interactions with your environment).