The dragons were genetically (or magically) engineered to be a slave race.A rider can survive without his dragon, but when a rider dies, his dragon also dies. There is NO WAY something like this could have evolved naturally.
This only happens with bonded dragons however. The existence of wild dragons indicates that this is the result of the bond, not the species.
Brom can teleport.How else was he able to get to Gil'ead from almost the Beor Mountains without Saphira, who was already in Gil'ead?