Phylogenetic systematics, a.k.a. evolutionary trees :
Using trees to make predictions about poorly-studied species: A new drug
Phylogenies also allow us to generate expectations about the characteristics of living organisms that we have not yet studied. For example, scientists discovered that the Pacific Yew produces a compound called taxol that is helpful in treating certain kinds of cancer, but it was difficult and expensive to get enough of the compound out of the tree to make its use broadly feasible. However, based on the evolutionary relationships among yew species, biologists expected that close relatives of the Pacific Yew might produce similarly effective compounds
Happily, they were right! They discovered that the leaves of the European Yew contain a related compound that can also be used to efficiently produce Taxol. Taxol is now widely available for cancer treatment.