It's also rare to just "discover" an entire continent that is basically free for the taking since Europeans annihilated native populations through disease and technological superiority.
Much of what makes America unique is tied to this essentially once in a generation event that will never happen again on this planet, a contingent confluence of Earth's parallel geographic and biological evolution... it's fairly easy to rebel or become a superpower when other powers have to contend with peer conflicts right on their borders. A break with England was inevitable why take orders from people an ocean away in the age of sail?
It's worse than that; within a few generations our linguistic and biological systems will begin to diverge under conditions with little cross-pollination and different selective pressures. We will become aliens in the sci-fi sense very rapidly if we attempt to create a foundation-like diaspora of settlements.
I think the skepticism warrants more work than that. Darwin's finches are an entry-level concept to learn when learning about biochemistry and genetics. Separate planets would act to separate groups into distinct genetic populations which would then have different selection pressures put upon them. Even without selection pressure, genetic drift in both populations would result in differences compounding over time.
Humans aren't the endpoint of evolution. Something will come after us, and if we're spread out on a ton of planets, there would need to be explicit counteracting forces (genetic modification, tremendous volumes of interstellar human travel, etc.) to make sure whatever comes after us is uniform among our interstellar backyard.
No, most genetic drift that ends up being reflected in wildtype populations happens during those periods, because small errors taking up a sizable percentile of the allelic distribution is easier when there are less alleles.
When it comes to neutral mutations, we can literally see constant variance creation in plenty of non-coding areas of DNA over time.
Drift occurs at a fairly consistent rate that reflects the intrinsic error rate of the particular replication machinery that a given organism uses. You can measure the statistical error rate of different ribosomal complexes.
Different planets are going to have different selection pressures. They'll have different conflicts. Different crises. Different cultures. Different reproductive preferences. Imagining that these populations will converge on the same wildtype by sheer chance is lunacy.
Much of what makes America unique is tied to this essentially once in a generation event that will never happen again on this planet, a contingent confluence of Earth's parallel geographic and biological evolution... it's fairly easy to rebel or become a superpower when other powers have to contend with peer conflicts right on their borders. A break with England was inevitable why take orders from people an ocean away in the age of sail?