I always figured mermaids would be more like cetaceans than fish. Having scales on their tails would be inaccurate, but breathing air, growing hair, and having a horizontal tail fluke are all in-line with mammals/cetaceans.

Not to mention, it would mean mermaids would have regular genitals (albeit hidden inside a genital fold, to increase hydrodynamics.) It would also mean they’d have normal placental pregnancies.

So that’s, uhh… a thing to consider.

Umm man had sex with a fish and some very very unlikely offsprings endedup being the humans of the sea and so they dominating it.

What evolution do you need?