The typological theory of species classification, which has few adherents today, distinguishes species solely on the basis of observable physical characteristics, such as plumage color, adult size, or dental structure. However, there are many so-called “sibling species,” which are indistinguishable on the basis of their appearance but cannot interbreed and thus, according to the mainstream biological theory of species classification, are separate species. Since the typological theory does not count sibling species as separate species, it is unacceptable.
The reasoning in the argument is most vulnerable to criticism on the grounds that
A． the argument does not evaluate all aspects of the typological theory
B． the argument confuses a necessary condition for species distinction with a sufficient condition for species distinction
C． the argument, in its attempt to refute one theory of species classification, presupposes the truth of an opposing theory
D． the argument takes a single fact that is incompatible with a theory as enough to show that theory to be false
E． the argument does not explain why sibling species cannot interbreed