If evolution drives species apart in qualities, then why are evolutionits so focused on the similarities of the species as proof for common descent? It may drive them apart in physical characterists to some small degree, but genetically they share much of the same characteristics, as we are constantly being told.
Because common descent is a key claim of evolution and similarities, especially certain genetic similarities, forming a tree-like structure are evidence for it.
If evolution did not drive species apart we would have only one genome and one species. Only a small difference in genome can result in the difference between related species, in which case the majority of the genome will remain the same. The more different the genomes, the farther apart two species are. The more similar the genomes the more closely related. This is a direct result of the 'push' to speciation inherent in evolution.