Agreed, but what makes one species able to survive when the other doesn't?
Well, take for example the Manx breed of cat - they are completely tailless. They survived because the Manx people (Isle of Man) LIKED THEIR LOOKS and selectively bred them - thus, on that island this breed became dominant. Note that they are tailless because of a genetic variation, not because the Manx people cut off their tails.
Another example would be many species on the Galapagos Islands; one example is the only known species of Iguana which deep sea dives to find sea bottom plants to feed on. They have not only behavioral differences, but also physical adaptation.
The Komodo Dragon is another case: They might well be extinct if there were another top predator on their islands, but they turned out to be the top of the predatory chain for now. Human intervention, or new invading species, could change that in the future.