Bog-standard evolution is perfectly capable of large qualitative differences. Animals couldn't fly until they did. Land animals didn't exist until they did. Multi-cellular beings weren't until they were. We can empirically see this, so your question should be flipped to "how can we explain such developments within evolution?" (We can, but that's a different discussion).
I guess where you stand in this debate depends on how you define “qualitative”.
One could argue that being able to fly or not is a matter of quantity, not quality. But unless you’re being deeply philosophical I admit this is a silly argument to make.
