News story here.

A key to how evolution works was that the genes for this, when somehow accidentally expressed, just suddenly popped a whole fin out there. There wasn't a little stub, then another generation with a longer stub, and so on until you had a fin. Would be interesting to see if one or more offspring had this.

There's a lot of surprising "suddenness" to evolution (see the link in my previous post) that isn't thought about much by the average person.

This dolphin could grow a fin in one generation because the genes for that back limb were there since the dolphin's ancestors were fish. For a while they worked into legs, then probably into fins, and then faded out. But the blueprints in their basic forms are there. It would be a lot harder to get a dolphin to grow a real leg--I'm sure the back fin is a more recent development--but not impossible.