Under what conditions might the morphological species concept be used instead of the biological species concept?

a. it is clear that one species evolved into another, based on fossils.
b. all members of one population can successfully interbreed with another population.
c. the fossils of the species are similar, but evidence for reproductive isolation between them is unclear.
d. two closely related species can hybridize but the offspring are sterile.
e. a particular "species" is sexually reproducing.