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

It is likely that the genomic sequences that were used did not enable to properly observe the evolutionary relationships among the species analyzed

Explanation:

The homology level between the genomic sequences may be too low to separate species that are phylogenetically related. In these cases, it is necessary to make a genetic analysis of the sequences that are used before the phylogenetic analysis