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Originally Posted by reprise85
In a way it's actually a bit more complicated if this is the way it works, because adaptations don't occur unless they are immediately advantageous. Evolution doesn't think ahead. So unless the gradual adaptations had some kind of advantage, they wouldn't happen. For example, birds just didn't develop wings out of nowhere, of course. It had to take 1000s of years to go from reptile arms or whatever to wings. The hypothesis is that the intermediate step - feathers - were originally used for warmth. Only then could feathers even be a proximal enough mutation to be selected for.
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For a while in biology, it was debated how something as complex as an eye could evolve, when the probability of it seemed really low if mutations are random. Richard Dawkins used a
computer simulation to demonstrate the idea that mutations aren't completely random, which makes the probability of complex adaptations a lot higher. Something that functions 10% like an eye is still better than no eye at all, so that would be selected for, and then mutations that further improved the eye would also be selected for. The thought experiment was sort of like: it would take an insane amount of time for a it to be likely for a bunch of monkeys on typewriters to type the line from Shakespeare "Methinks it is like a weasel," but if you were to make the process iterative so that every correct character would stay and be built upon, you can get the phrase in just a few generations.
I guess a similar idea is at play in nature. Only difference is that, whereas the Dawkins experiment has a target goal, natural selection doesn't, because the environment is always changing. Something like an eye, I can imagine, would be advantageous in almost any environmental condition (unless you were some cave organism that lives in total darkness or something). I never thought of the idea that these complex adaptations come about because the intermediate forms are advantageous for something else other than what the adaptation will later be (your bird wing example), though. That's really interesting and it makes sense.