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For some species the Darwin's theory evolution makes perfect sense. I can easily imagine how, for example, the giraffe has evolved to its current appearance: the natural selection was favoring individuals that could consume more vegetable food from trees using longer necks, and some individuals were getting at birth necks longer than average by pure genetic randomness and the long neck trait was being propagated to descendant individuals by means of genetic inheritance. I have no problem with understanding this kind of evolution.

Now let's have a look at the bat and its relatives. The bat is one of the few mammals that have something to do with flying and the only one that took flying to the bird level. Paleontologically, first mammals date to the dinosaur era and initially looked similar to the present-day shrew (which looks much like a mouse). The question is: how in the world prehistoric mouse-like creatures could grow wings over time? It impossible to believe that some mouse-like individuals were getting wing-like limbs by mutation and the "wings" were growing out accompanied with the knowledge of how the "wings" can actually be used. Ok, then maybe first wings were tiny moth-size wings and then grew larger? But where natural selection would come into play in this case? Such mouse-like individuals would have no advantage over their wingless relatives and thus would not be able to transfer those wing-growing genes to their descendants, quite the contrary, such individuals with useless mutations that interfere with their ability to walk would be suppressed by natural selection and therefore "weeded out".

So what is the story behind the bat's wings and is the Darwin's theory really able to support it?

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    $\begingroup$ Darwin's theory was flawed and that's not a secret, but the contemporary theory of evolution is about as accepted as it gets. Species or traits which pose a challenge to it are nowadays more commonly regarded as a case which will be solved sooner or later (possibly by fossil findings) rather than a potential proof against the theory. $\endgroup$
    – Armatus
    Aug 1, 2012 at 19:52
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    $\begingroup$ Please don’t cross-post. – If you’re unsure which site to post on, discuss this on the meta sites. FWIW Skeptics is a better suited site. A biology website is probably not the best place to question the basic tenet of biology, much like the Christianity Stack Exchange will not accept discussions about the non-existence of god. $\endgroup$
    – Konrad Rudolph
    Aug 1, 2012 at 21:26
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    $\begingroup$ @KonradRudolph: the cross-post looks like it may be closed as off-topic there. $\endgroup$
    – Mechanical snail
    Aug 2, 2012 at 1:25
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    $\begingroup$ sweary.com/dailyfun/wp-content/uploads/giraffes.jpg $\endgroup$
    – wim
    Aug 2, 2012 at 1:37
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    $\begingroup$ @Mechanicals How do you figure that out? Two mods (me included) have commented saying that it’s on topic. $\endgroup$
    – Konrad Rudolph
    Aug 2, 2012 at 6:47

3 Answers 3

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Take a look at this little fellow:

Flying squirrel

It's a flying squirrel — a shy little nocturnal rodent which lives in trees and, despite its name, does not actually fly. It does, however, have a skin membrane called a patagium between its fore and hind limbs which allows it to glide from tree to tree and thus evade ground predators.

It's not hard to see how the flying squirrel's patagium may have evolved: after all, ordinary squirrels, to which the flying squirrel is indeed related, also spend most of their time in trees and avoid the ground, often performing quite impressive leaps to cross from one tree to another. With sufficient pressure to minimize time spent on the ground, any little morphological changes that allowed longer leaps would be favored by natural selection.

Indeed, there are plenty of other groups of mammals which have independently evolved very similar adaptations to gliding. Given how many small arboreal mammals there are, this is perhaps not surprising. What's special about bats is not the fact that they possess flight membranes — it's that they're the only group of mammals, so far, to have taken the next step to actual powered flight.

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    $\begingroup$ Gotta love the way you started your answer. Thanks. $\endgroup$
    – Desmond Hume
    Aug 2, 2012 at 11:15
  • $\begingroup$ So basically, once trees evolved, birds (and bats) were inevitable. $\endgroup$
    – OldCurmudgeon
    Aug 3, 2012 at 1:12
  • $\begingroup$ I think sugar gliders (and the like) are different in that their skeletons dont indicate that they are gliders. The bone structure of bats have clearly evolved i.e. in forming elongated fingers to fit their wings. $\endgroup$
    – bondonk
    Jul 21, 2016 at 13:28
  • $\begingroup$ try "flying" lemurs then, sugar gliders would the very start of the adaptations. you can also look at onychonycteris a late bad ancestor with short fingers compared to modern bats but far too long to serve any other function. $\endgroup$
    – John
    May 9, 2018 at 0:07
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Check out this famous paper by Gould and Lewontin, the Spandrels of San Marco. It's essentially a criticism of your question as it's not really the right question to ask. You're engaging in what they call "adaptive storytelling". The truth is that barring fossil intermediaries it's almost impossible to infer how a species transitioned from primitive ancestors. Secondly you're assuming constant selective pressure over time. It certainly can be the case that traits evolve in certain environments but are conserved over time despite no longer serving their original function (why do flightless birds have feathers still? etc.).

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  • $\begingroup$ Penguins would be pretty cold without their feathers. $\endgroup$
    – Bob says reinstate Monica
    Jul 22, 2017 at 14:01
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Perhaps you didn't realise that bats' wings are equivalent to the front limbs of other mammals? They aren't extra limbs, just adapted ones, and the wings are formed from flaps of skin between the 'fingers'.

Taking that into account, I don't even see how anyone can think this is a challenge to the theory of evolution. It's obvious that increased skin webbing between fingers, eventually combined with increased finger length, provides an adaptive benefit to tree-dwelling small mammals. Initially it gives increased air resistance to allow controlled falling, then to allow gliding (like in flying squirrels), and in the most extreme cases to allow flying.

These kinds of questions are always about a failure of imagination, or a misunderstanding of the basic facts, and never about the theory of evolution being wrong. (edit: that doesn't mean the theory of evolution can't be challenged - it is a theory - but that this isn't a valid way to challenge it).

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    $\begingroup$ +1, I agree completely with you but I would remove or rephrase the last sentence. The theory of evolution is our current accepted model for the evolution of species. It is a scientific theory and hence it is falsificable. Maybe 500 years in the future strong evidence will have shown up that will make us rethink our current theory of evolution, much like some of Darwins' original points are now being refined. Assuming that any questioning of a scientific theory stems from misunderstanding it and that the theory cannot be wrong makes it become something more like a religious dogma. $\endgroup$
    – nico
    Aug 2, 2012 at 6:29
  • $\begingroup$ By 'equivalent' you mean 'homologous', right? $\endgroup$
    – Mechanical snail
    Aug 2, 2012 at 7:00
  • $\begingroup$ @nico I certainly didn't mean the theory of evolution is immune to testing or falsification, but that saying 'I can't imagine how X trait evolved, therefore the theory is wrong' are not valid challenges to the theory. My answer was more of a grumpy tirade than a proper answer :) $\endgroup$
    – Rik Smith-Unna
    Aug 2, 2012 at 8:43
  • $\begingroup$ @Mechanicalsnail yes, but given the asker thought bats' wings were an extra limb, I went for non-technical vocabulary. $\endgroup$
    – Rik Smith-Unna
    Aug 2, 2012 at 8:45
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    $\begingroup$ @RichardSmith-Unna: I think the technical term for the OP's line of reasoning is Argument from Personal Incredulity. $\endgroup$
    – Oscar Bravo
    Jul 27, 2016 at 12:02

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