The normal explanation is that genetic variation resulting from mutations means that some skin forms small webs between arm and side, and this slight difference allows a squirrel to jump a little further, leading to increased access to food and increased ability to evade some predators.
This is one of the kinds of explanations that evolutionists make about how complex features could have formed, that is the most unsatisfying to me.
In your scenario the creature obtained a small mutation (I assume it must be quite small, because we don't see many of these gross mutations forming in all of our observations of animal populations).
Now the amount of illogical assumptions just continue to grow and grow in this scenario:
-- The animals that preceded this mutation apparently were finding food just fine, so were they really that handicapped compared to this new mutant?
--The mutation would have had to have been exactly symmetrical on this mutants body to be of any use at all.
--How much further could a squirrel with a little flap of skin jump compared to one with none, a few inches?
--Were there other mutations also going on within the population that were giving other individuals advantages, that the one with the flap of skin didn't have? Or did every other individual stop gaining any beneficial mutations during this time that helped them in another way? In other words, that one mutant individual had one kind of benefit, but perhaps others had bigger stomachs, or bigger jaws, or any of thousands of other advantages that this guy didn't have. Or are we to believe all of these other mutations and selections are happening consecutively but not concurrently?
--This zero point mutant (the original) gives birth to a second with a similar mutation. Once again, this small advantage is trumping all other advantages within the population, even though there could be some things this mutant has that are not good at all, like the wrong color fur?
--How many generations do you want to continue down the line of this squirrels ancestry before the next mutation that improves upon this one occurs? Ten generations, 20, 100? How long before this next one pops up which does exactly what the previous one did, only better? Not a flap of skin elsewhere, or one that makes the skin mutate smaller again, or what have you, but one that once again directly benefits this previous one?
--Could the descendant of this squirrel get a different mutation, like say better camouflage which could help it to survive, but not for the reason of jumping better, but because it hid better? Now are we selecting for both at the same time, or for one or the other, or does he get a slight advantage this generation for his camouflage, and the next generation for better jaws, and the next generation, for better hearing, etc,etc..
--While we are sifting through these generations upon generations of selecting, are the needs for survival staying constant? Is one generation struggling for more food, while another generation is struggling to hide from predators, while another is struggling for water, while during another generation the food source has changed from being in the trees to being on the ground?
--Did the guy who got the first beneficial mutation (beneficial mutations don't happen very often right, much less often then detrimental ones, correct?) also happen to be born with a parasite in his intestine which caused him to die before he could reproduce?
--Or was he at the wrong place at the wrong time and happened to have gotten picked off by an eagle when he was only 6 months old? Whoops one good mutation that could have really gotten our team off to a good start, and now we have to wait ANOTHER 500 generations for the next beneficial mutation.
--If the first zero mutant of such good fortune didn't happen to get picked off by the eagle, but his son did, we are right back to the same problem again right?
--Can we list ANY examples of beneficial mutations that we have observed in nature as a starting point, that has the potential to give one individual a bio mechanical advantage which could lead to the creation of a new trait? ONE? Ever?
Now I honestly believe I could go on and on with the logical difficulties your theory faces, but the problem is that your side wants to brush EVERYONE of these difficulties aside, and claim it is the ID's or creationists who are living in a faiy land void of empirical evidence.
You will say to the ID side, well how do you account for this, and for that, and so on...and yet you can't account for any of these issues, let alone all of them. But you STILL claim to have science on your side.