That’s an interesting question. Generally speaking, evolution occurred from salt to fresh – the seas were salty before there were land animals, for example. Whales and dolphins are actually animals that evolved from land animals, which came from the (salty) sea originally. (That’s not your question, but it’s an interesting thing anyway).
The way we usually figure out how things evolved is by looking at physical features of an animal, and (now) by sequencing their genomes. If there is a particular feature shared by modern and ancient animals (something we can see in fossils), and we can determine what environment the ancient animal lived in, we could say that the modern animal evolved from the ancient one – perhaps from fresh to salt. What’s perhaps more clear would be genome sequences from related modern animals, from which we could infer a last common ancestor. There’s not a lot of difference between most fresh and salt animals – some fish go between the environments during their lives – so I can’t say that there’s one particular feature or part of the genome that would indicate this.
That’s an interesting question. Generally speaking, evolution occurred from salt to fresh – the seas were salty before there were land animals, for example. Whales and dolphins are actually animals that evolved from land animals, which came from the (salty) sea originally. (That’s not your question, but it’s an interesting thing anyway).
The way we usually figure out how things evolved is by looking at physical features of an animal, and (now) by sequencing their genomes. If there is a particular feature shared by modern and ancient animals (something we can see in fossils), and we can determine what environment the ancient animal lived in, we could say that the modern animal evolved from the ancient one – perhaps from fresh to salt. What’s perhaps more clear would be genome sequences from related modern animals, from which we could infer a last common ancestor. There’s not a lot of difference between most fresh and salt animals – some fish go between the environments during their lives – so I can’t say that there’s one particular feature or part of the genome that would indicate this.