Why was molecular testing necessary to establish that species Z was closely related to Butanol Curus? In other words, why couldn't we just stop after the first three tests?

Thanks

Because it is only possible to be 100% sure that two species are closely related, in genetic terms, when the DNAs of these species can be analyzed. This analysis is only possible through molecular tests.

Explanation:

Molecular tests are bikini techniques that can analyze species at the molecular level, more specifically, these tests can deeply analyze the DNA (or RNA and proteins) of a species giving precise results on the molecule and allowing scientists to study it deep.

In studies, where the scientist wants to know how the species are genetically related, it is common for molecular tests to be introduced, as a way for the scientist to be 100% sure of the relationship between the species.