A couple of thoughts I just had on evolution
Sea snakes: Given that sea snakes never come on land why are there no sea snakes with a gill system? What possible advantage could there be for them to remain air breathers? Perhaps the same question could be applied to dolphins/whales. Perhaps being warm blooded might be a problem, that is, the amount of oxygen required would exceed what gills could deliver.
One would think at least some sea snakes would breathe via gills.
Another thought was the variety of animals that live in the same environment. It is obvious from how prolific some are compared to others that some are far more suitable to the environment. Shouldn't evolution mean that the number of species would be greatly reduced?
I think the sea snake not devloping gills is perhaps similar to the lizard not getting to the snake skull/jaw structure. The legless lizard has taken many characteristics of the snake but can't make the final jump. Many sea snakes have the paddle tale and their scale structure (why they can't move on land) stream lines them for going through water but they can't make the final jump to gills.