bertrand@eiffel.UUCP (Bertrand Meyer) (04/15/91)
Here are some extracts from an article entitled ``Dinosaur Dilemmas'' by John Maynard Smith, Professor of Biology at the University of Sussex, appearing in the latest issue of the New York Review of Books, dated April 25, 1991. The article is a review of two recent books on dinosaurs. --------------------------- Begin quote ------------------------------ The interpretation of [dinosaur] fossils is primarily aimed at recognizing anatomical similarities and differences, and arranging the specimens into a natural classification. [...] Today, a ``natural classification'' is usually taken to mean one that reflects evolutionary history. If species are grouped together in a ``taxon'' (that is, a genus, family, order, or what have you), the implication is that the taxon includes all species, and only those species, descended from a common ancestor. [...] It is clear from reading ``The Dinosauria'' [a book published by the U. of California Press] that an important change for the better has taken place since I was a student, although the new dispensation too will become an abuse if we do not watch it. The name of the change is ``cladistics.'' This is one of two very different approaches that have been made in recent years to devise a rationale for classification. The first was numerical taxonomy. Robert Sokal and Peter Sneath, the founders of numerical taxonomy, took the simple way out. If there is no good reason for preferring one trait to another, let us, like good democrats, treat all traits equally. When classifying a set of animals, write down everything about them, in as mindless a way as possible. Then write a computer program that will group together those species that are alike in most respects. It was a method only possible with the advent of computers. It had the great merit that there was no damned judgment about it. In practice, however, numerical taxonomy has been largely replaced by cladistics, essentially, I believe, because Sokal and Sneath insisted that classification should be independent of evolutionary considerations. Cladistics was invented by an East German taxonomist, Willi Hennig. [...] There are two components to the method, one substantive and one semantic. The substantive component concerns which traits are important when classifying, if one wishes one's classification to reflect evolution. The rule is that one should use ``shared derived characters'': there are some impressive Greek words for these that I won't bother with. For example, zebras and horses have a single toe, whereas opossums and humans have five toes. To have five toes is not evidence of close relationship, because it is the primitive condition in land vertebrates; but having a single toe is good evidence of relationship, because it is a recently evolved state. The hard question, of course, is how to decide which is the primitive and which the derived state. If we had a perfect fossil record, there would be no problem: but if we had a perfect record, classification would be trivially easy anyway. [...] [The] other, semantic component of cladistics [...] asserts that any named taxon - for example, Eutheria (placental mammals), Dinosauria, Lacertilia (lizards) - must be monophyletic; that is, it must include all, and only, the descendants of a single common ancestor. Notice that, according to this requirement, Pisces (fish) is not a valid taxon, because it does not include the land vertebrates, which are descended from fish. Consistent with this view, the editors include the birds among the dinosaurs, as they must on cladistic grouds, because the birds are thought to be descended from a particular group of small carnivorous dinosaurs. --------------------------- End quote -------------------------------- -- -- Bertrand Meyer Interactive Software Engineering Inc., Santa Barbara bertrand@eiffel.uucp
chl@cs.man.ac.uk (Charles Lindsey) (04/15/91)
In <527@eiffel.UUCP> bertrand@eiffel.UUCP (Bertrand Meyer) writes: >Here are some extracts from an article entitled ``Dinosaur Dilemmas'' >Consistent with this view, the editors include the birds among the >dinosaurs, as they must on cladistic grouds, because the birds are >thought to be descended from a particular group of small carnivorous >dinosaurs. Ergo, mammals are dinosaurs also, and hence so are we.