WW wrote:
"Second, genera are primarily defined on the basis of monophyly, evidenced from analysis of various characters, not on the basis of external similarity. The characters that identify various monophyletic groups are not necessarily characters that are easily used for field identification (e.g., cranial bones etc.)"
From his posts and from our past interactions, WW subscribes to the Hennigian definition of monophyly, which is different than the Darwinian's definition. According to Hennig's definition, a monophyletic taxon consists of a single ancestor and all of the descendants of this ancestor. According to the Darwinian definition, a taxon is monophyletic if all of its members share a nearest common ancestor, whether or not all members are included in the taxon. For example, Class Reptilia is considered monophyletic by the Darwinians even though some of the descendants of its common ancestor, namely the birds and the mammals, are excluded from Reptilia. Reptilia is not monophyletic according to the Hennigians because it does not include all descendants of its common ancestor.
Since the discovery of the RNA/DNA based universal genetic code, it is clear that all life on earth is almost certainly descended from a single common ancestor, in which the genetic code evolved. Therefore all life on earth, including the extinct species, form a monophyletic group, either using Hennig's definition or the Darwinian definition of monophyletic. All life on earth therefore can be classified in the same genus, if one were to define a genus primarily on the basis of "monophyly."
It is quite obvious that classifying all life on earth in a single all inclusive genus is not very useful for biologists or the general public. Clearly there are organisms that are more closely related to each other than they are to other organisms. For example, it does not take a biologist to realize that trees are probably more closely related to other trees than they are to, say, a worm or a bird. Therefore there must be additional taxonomic categories other than genus to construct a useful classification of all life on earth. That is why the Linnaean system is so useful, because it uses more than just a single taxnomic category to rank organisms according to their different degrees of relationships. Closely related species are put into the same genus, whereas more distantly related species can be put into different genera. Different genera can be placed in different families if they are not closely related. Using the Linnaean ranks, biologists have been able to classify the millions of extinct and living organisms on earth for hundreds of years.
As WW points out, genera are defined primarily on the basis of monophyly. In fact all taxa regardless of rank (kingdom, phylum, class, order, family, genus and species) must be monophyletic. Clearly then, monophyly alone is not useful in deciding whether two organisms are classifiable in different kingdoms, different families or different genera. Therefore biologists through the centuries have classified organisms according to the degrees of their evolutionary relationships. Until the latter half of the 20th century, the only evidence for such relationships have come from a study of their morphological differences. With the increasing use of molecular techniques, some of the traditional taxa that are grouped on the basis of morphology have been shown to be polyphyletic, i.e. their similarities are the result of evolutionary convergence, rather than inherited from a common ancestor. Such taxa are of course unacceptable to either the Darwinians or the Hennigians and they have been dismantled and the species within them reclassified.
Some taxa, however, are neither polyphyletic nor are they monophyletic according to Hennig's new definition of monophyletic, although they are monophyletic according to the Darwinians. These taxa, dubbed "paraphyletic" by the Hennigians who are intolerant of them, are the major source of disagreement among different schools of taxonomists today. Most (but not all) taxonomists who adhere to cladistic methodology are intolerant of paraphyletic taxa. The Darwinians tolerate them because these taxa are descended from the same common ancestor, and because they are most similar to each other and to their common ancestor morphologically. A very good example is Elaphe. The species classified in Elaphe are all morphologically very similar to each other and to their common ancestor. This is a slowly evolving group that should be retained in the same genus. Yet because a member of Elaphe has migrated to the New World and given rise to a number of other more evolutionary divergent species which have been classified in genera such as Lampropeltis, Cemophora, Stilosoma, Bogertophis, Pituophis and Arizona, Elaphe is “paraphyletic” according to the Hennigians and therefore it must be dismantled. It is not easy to splinter a morphologically homogeneous genus such as Elaphe and transfer the dismantled pieces into nearly a dozen smaller genera which are morphologically indistinguishable from one another. Besides, it does not do any good for either the general public or the biological community to split Elaphe into so many pieces. Information retrieval will be more difficult. Communication will also be harder. The new classification also obscures the fact that Elaphe is a slowly evolving genus that has remained largely unchanged since the Miocene. Some Miocene fossils, for example, can be identified as being conspecific with living species. For these reasons, it is best that the scientific community ignore the cladists’ proposal to replace paraphyletic taxa with contrived taxa that are morphologically undefinable and/or indistinguishable from each other. Contrived genera such as “Pantherophis”, “Pseudelaphe” and “Euprepiophis” are morphologically undefinable and the proposal to resurrect them should be ignored. These species that had been transferred to these contrived genera by Utiger et al. should instead remain in Elaphe to indicate that they form a monophyletic group and that they have diverged very little from one another and from their common ancestor, notwithstanding the cladists’ intolerance of taxa that do not conform to Hennig’s non-traditional definition of monophyletc.
