As for small changes that make creatures "fly", or at least glide: there's a species of snake in SE Asia that has a flattish body, and is able to flatten itself a fair bit more - flexible rib joints, I guess. It will drop from a tree, 10 meters up or so, and by flattening and writhing make itself into enough of an airfoil to land 12 or 15 meters away from the tree. A pretty handy escape mechanism, or even a hunting move, eh? And with only minimal modification, at least to a casual observer like me. Now, why can't tiny improvements in that trait be passed on, and lead to better fliers? Flatness doesn't seem that useful to a snake otherwise, unless it's for sneaking under doors....
I'll dig out references/links if anyone wants them.
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