The other basic way of looking at phylogeny is the old standby, structural comparison, in which one looks for similarities and differences, called characters or features,(Walking dinosaur costume) between the designs of species, or their morphology. (Animatronic dinosaur )The more similarities and fewer differences, the closer the relationship may be. But there is more than one way for two animals to become similar. The first, of course, is to develop from the same stock. A second way is by developing similar adaptations independently, under similar selective pressures. This is called convergence when it happens among distantly related animals, parallelism if t’wo closely related taxa do it. Take the cat, the dog, and the thylacine. The thylacine, or Tasmanian “wolf,” looks a 1ot like a dog. In fact, the shape of its skull is remarkably doglike. But the details of the skeleton and soft tissues show that thylacines are really marsupials, like kangaroos. Thylacines and canids look so alike because they hunt in similar ways, and they have converged. Cats, on the other hand, are close dog relatives, but they do not look as doglike as thylacines because their body form evolved for a different, stealthier method of hunting. Such a change in form away from a common oattern is called divergence. Then there are reversals, in which species evolve adaptations that look like ones earlier members of the group had lost.
Convergence, parallelism, divergence, and reversals can really confuse the phylogenetic investigator. Realistic dinosaur model The modern way to sort out these problems is based on cladistic principles. A clade is a group of animals that share a corrrmon ancestor. Such a group is called monophyletic. A group of animals that has multiple ancestors is polyphyletic, and is not a true phylogenetic clade. Of course, all this is relative-ultimately, every living thing shares a 3.6-plus-billion-year-old single cell as the first great-grandparent many times over.