Your video doesn't seem to disagree with anything I posted. The evolution of the patagium in the squirrel would have had to have existed for hundreds or thousands of generations before it granted the squirrel any evolutionary advantage. Natural Selection would not account for that.
Yeah it does.
It's been argued ad nauseam, but whenever someone points to, say, the evolution of a fully-functional wing attributed to a creature which is supposed to have evolved from a bipedal creature into a flighted creature, it usually gets explained via creating reasons why a species would evolve and hold onto a useless or near-useless vestigial wing for the thousands or millions of years required for the vestigial wing to become a fully-functional wing, all the while evolving a slightly more efficient wing every generation (all the while said vestigial wing provides no boon for the creature).
This is a reason why a species would hold onto a vestigial wing - because it helps the species propagate, and/or it doesn't hurt them.
What you then said as a followup is basically, "Okay, but how did the squirrel get to where it is NOW?"
The answer to that would be, "Presumably, by a similar process".
The evolution of the patagium in the squirrel would have had to have existed for hundreds or thousands of generations
Flaps of skin. Are they that uncommon to mammals? Not really. Consider this.
For some reason, a given population of squirrels has a very slight amount of extra skin between their limbs, probably due to some random mutation. It doesn't give them an evolutionary advantage, but it doesn't hurt them either, so the trait persists over time.
Eventually, over time, over enough generations and mutations, some squirrels have slightly larger flaps, and some have slightly smaller flaps, all within a given distribution of genetic probability. None of this matters because it still doesn't affect the survivability of squirrels or the passing on of their genes.
Until it does.
Maybe some squirrels eventually see large flaps as sexually desirable and thus squirrels with bigger flaps are more likely to mate (e.g. peacocks).
Maybe some squirrels are better able to keep themselves cool due to more blood vessels in their flaps exposed to the air like a radiator, and thus more of them survive and mate (e.g. Asian vs African elephants)
Maybe the larger flaps confer some sort of slight boost in jumping and gliding ability, and thus more of them survive and mate (e.g. flying squirrels)
Whatever the reason, there was some evolutionary pressure for the flaps to get bigger and bigger, because that's what happened. Just because you don't know or can't imagine what the reason was, doesn't mean there wasn't a reason.
before it granted the squirrel any evolutionary advantage.
That doesn't matter. As long as there is no pressure to kill that trait from a population, then it will still persist and change too. There is both positive and negative selective pressure.
Natural Selection would not account for that.
Yes it does. If some unique trait makes a certain segment of a population breed more successfully and pass on its genes, then that trait will be passed on. If having the trait means you're less likely to live or mate, then that trait dies out. It happens. This is an observed phenomenon. A trait that is present in modern flying squirrels didn't always necessarily have to have fit the use that it holds today. Like I explained earlier, there are many possible reasons why a flap of skin might turn out to be useful, which would lead to selective pressure to make it bigger, which leads to those squirrels eventually using it for other purposes like gliding, at which point there are more selective pressures applied to that specific use too.