After all, just a SMALL change means that cousins can't BREED!
In the south they do all the time. They even marry. I think you mean that an alligator can't breed with a whale.
Since it would be under the water, you can never know for sure. They just can't reproduce.
The existance of halibut proves that cousins can breed and reproduce since both eyes are on the same side of their heads. (its more rare in the south)