Quite. Mutations occur. Those that give an advantage or are neutral will get passed on. Those that are negative i.e. provide a disadvantage, don't. Advantage and disadvantage, in this context, mean how mutations aid the passing on of DNA. That's it.
But there are limits to what mutations can achieve, which explains the many design faults in organisms, if you assume design. I don't think any mutation could give a vertebrate six limbs instead of the standard four, so, as I said earlier, if humans were to have wings, we'd have to do without arms, which would be a retrograde step, if you ask me.