The typological theory of species classification, which has few adherents today, distinguishes species solely on the basis of observable physical characteristics, such as plumage of color, adult size, or dental structure. However, there are many so called "sibling species", which are indistinguishable on the basis of their appearance but can not 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.
Conclusion- the typological theory is unacceptable because it categorizes "sibling species" as the same species but mainstream theory says they are separate.
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 - incorrect; is wrong since you don't have to evaluate every aspect of a theory to conclude it's unacceptable. One bad part can ruin it!
(B) The argument confuses a necessary condition for species distinction with a sufficient condition for species distinction- no conditional statements here
(C) The argument, in its attempt to refute one theory of species classification, presupposes the truth of an opposing theory - Correct; the argument assumes that mainstream theory is correct.
(D) The argument takes single fact that is incompatible with a theory as enough to show that theory to be false - incorrect; The issue is that citing a single counterexample to an absolute rule is an acceptable way to argue -- it's not a flaw!
(E) The argument does not explain why sibling species can not interbreed- irrelevant
Answer C
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