Agreed, there is no way to know the exact degree of differences between the two populations (although any geneticist would reject the notion of primarily unselected genetic drift in the example I described. I specifically outlined strong selective pressures in the environments). But at least we agree that they would not continue to be identical, and that the environmental pressure, over time, would also show up in genetic drift between the populations (there are surprisingly many blank-slaters who would disagree even with this premise).

We have tools in modern science to attempt to differentiate between different underlying components of genetic variation vs environmental variation (identical twin studies, adoption-at-birth studies, general population studies, longitudinal analyses, etc). In lieu of a 1000-year experiment (which we cannot recreate), we can make predictions. If factor X was primarily genetic, we would expect to observe A, B, C. If factor X was primarily environmental, we would expect to observe D, E, F. Then, we can basically incorporate various studies and results and do gradient descent, and observing which (means squared error) is smaller.

None of this is groundbreaking stuff, I know. But I have read arguments from both the primarily environmental camp and the primarily genetics camp, and one of them has to do a lot more mental gymnastics explain the differences between obvious predictions (under this paradigm) and the observed data in the real world.

> I specifically outlined strong selective pressures in the environments

No, you didn't, because humans behave quite differently to your average animal. The only strong selective pressure you described was climactic: everything else can be mitigated with straightforward tool use. (And heck, going "oh, our skin is getting sunburned, let's be nocturnal instead" might cause your A-islanders to develop paler skin.)

You seem to be confusing "observed data in the real world" and "things that seem obviously true". If you pit your "obviously true" against other people's "obviously true" in your own head, of course your own "obviously true" is going to be the victor.