There are several causes for a lack of the ability to inter breed, but when this happened there is no new species.
Wrong. The *definition* of a biological species is an interbreeding population. When the population splits and no longer interbreeds, you have two populations that can adapt independently.
IMO, if there some intermediates, they would show them, but that is not important. You still can't explain how a nose can become a blowhole, and saying a land animal that goes into water to find food will become a sea animal is scientifically absurd.
Again, wrong. We see former land animals becoming sea creatures all around us: seals, walruses, hippos, etc. Now, it is *possible* their progression to the sea will be interrupted, but they are all mammals becoming more adapted to the sea and thereby sea creatures.
Going into the sea to find food makes it more likely that any adaptation to the sea will be enhanced and preserved. Again, the changes don't happen in one generation, or even one hundred.
And again, a blowhole is just a nostril at the top of the head.
I don't think that is it purpose, although what I said could be taken that way. I was really saying the DNA identifies animals of he same species,
It can be *used* that way because species separate, stop interbreeding, and thereby develop separately, acquiring different mutations and characteristics that we can look for in the DNA.