Final answer:
A species is a group of organisms capable of interbreeding and producing fertile offspring. Subspecies refer to variations within a species, but a single subspecies cannot exist independently; at least two are needed for comparison within the species.
Step-by-step explanation:
The concept you’re asking about relates to how species and subspecies are defined and recognized in biology. A species is essentially a group of organisms that are capable of interbreeding and producing fertile offspring. When members of a species become geographically or reproductively isolated, they can no longer interbreed with the main group, which may eventually lead to the formation of a new species over time.
As for subspecies, the term refers to variations within a single species that are not distinct enough to be considered separate species. According to the statement from Wikipedia, a single subspecies can't exist independently because for something to be considered a subspecies, there must be more than one distinct group of organisms within the main species that has minor differences, such as physical characteristics or genetic variations. If a species is recognized as having subspecies, there must be at least two different subspecies identified, because a single group that deviates from the species norm wouldn’t be classified as a subspecies unless there is another group to compare it to.