As we have just passed Hallowe'en, what better than a suitably-themed question with which to tax the SExperts? Apollyon obliged with this:
How did bat wings evolve?
A topical question in more ways than one, given the ongoing battle between science and creationism in certain parts of the world.
Indeed, when Apollyon added:
I ask because thinking about this matter, I couldn't imagine an intermediary stage between wingless rodent and bat that would confer clear survival benefits.
That's creationist thinking. Evolution is far more unpredictable than that. You cannot assume that any living being is the randomly-mutated best adaptation for its lifestyle. A so-called 'intermediary' stage does not exist because the generational changes are far too slight as to be distinct.
— Mu Beta
A mutation doesn't have to 'confer clear survival benefits', it just needs to not prevent the individual carrying it from procreating. There are many mutations which aren't exactly beneficial (ginger hair, for example1) but which still occur. Equally, I doubt you can think of all the possible habits and environments in which bat ancestors may have existed. Therefore our human imagination and knowledge cannot be a limiting factor on evolution.
Imagine a rodent that has flaps of skin so it can jump from higher in a tree, or jump higher off the ground... more insects are catchable, so they outcompete the others
— Not Him
I'd imagine the intermediary stage would be assumed to be something like flying foxes. That is, flaps of skin between arms and legs to allow gliding (right front leg to right hind leg)
Having agreed that evolution from rodent to bat is possible, it fell to Ste to provide a little in the way of explanation:
Flying squirrels represent an excellent example of what proto-bats might have looked like.
Bat wings are made up of the same bones that the human hand is comprised of, just longer with the skin stretched out. If you look at your hand and splay it out it is easy to imagine how a bat wing evolved seeing as it is pretty much the same structure. The same bones are also found in whale fins, the legs of horses, dogs, etc.
In fact, the evolution of flying rodents may have occurred more than once in the history of life on Earth. Racking his brain, pedro recalled...
I vaguely remember hearing that bats evolved twice, from two separate ancestors, so not all bats are related.
So, not only can evolution from land mammal to flying mammal be explained by evolution, it can be explained twice, and not an intelligent designer in sight...
For those wanting a deeper discussion of this and other issues relating to evolution, why not read Ste's excellent Guide Entry on The Theory of Evolution, or try searching the web for 'Pentadactyl Limb'. And if that throws up anything that has you baffled, simply head over to Science Explained and ask those pinnacles of evolution — the SExperts — to help you out.