Two populations of ciliated protists, Ciliatus morphus, are found in two separate ponds. Under a light microscope, individuals from both populations appear identical in size, shape, and arrangement of cilia. However, they differ in their preferred habitat: Population A lives in water of pH 5.0, whereas Population B lives in water of pH 7.5. When individuals from the two populations are mixed in the laboratory, they do not mate with each other, although they each undergo sexual reproduction within their own population. They also reproduce asexually by binary fission.
Compare how the biological, morphological, and ecological species concepts would determine whether these two populations belong to the same species. In your answer, also state the domain and kingdom to which these organisms belong and justify your classification using the characteristic features of the kingdom.