Did the saltwater fish swim into freshwater and adapt? Or did freshwater fish come first, while the ocean was young? Or is evolution a bunch of horse pucky?
Interesting question. I hope someone comes on here who knows the answer, but if I had to hazard a guess, I'd say probably salt to fresh. One the other hand, why did they have to start in one or the other, and not both at the same time?
Evolution from a single celled organizim is a bunch of crap but, Evolution with in a spieces still happens. Evolution is just addaptation but a fish is a fish and a human is a human and will stay that way till the dawn of time.