The correct answer is - species.
The extinct Caribbean monk seal was a species of seal that lived in a relatively small area in only one prat of the planet. Also, this species lived in waters that are not typical for the other seals. Because the Caribbean monk seal was isolated from the other species of seals, its genes over time became different enough from the other types of seals so that they can not mate and produce offspring between each other. While the isolation made it a unique type of seal, it was also very unfortunate because it meant that it can not be saved, at least in a mixed form, by mating with other seals, so this species of seal went extinct.
