Final answer:
Two populations are likely considered different species under the biological or phylogenetic species concepts, depending on reproductive compatibility and genetic lineage; they could be different subspecies under the morphological concept if differences are notable but reproduction is possible.
Step-by-step explanation:
Two populations can be considered different species under the biological species concept if they are unable to mate and produce fertile offspring. Under the phylogenetic species concept, they would be defined as different species based on their unique genetic history and lineage, rather than just reproductive isolation. That being said, if the populations differ significantly in morphology, but can interbreed or there's no information on their reproductive compatibility, they might be described as different subspecies under the morphological species concept.
The two populations are different species, under the biological species concept.
The biological species concept defines a species as a group of interbreeding organisms capable of producing fertile offspring. If two populations cannot interbreed or produce fertile offspring, they are considered to be different species.
For example, if two populations of birds have been isolated from each other for a long time and have evolved different characteristics, such as different coloration or mating behaviors, they would be considered different species under the biological species concept.