Let me start by saying that I am not on an anti-Hennigian crusade. The Hennigians are entitled to their believes of course, as are the pheneticists and the Darwinians. It is in fact the Hennigians who are imposing their views upon the traditionalists in their war against traditional taxonomy. A prime example of this imposition is their hijacking of traditional terms for their own use. The term monophyletic has never meant groups that consist of a single ancestor and all of its descendants. But the cladists insist that all other biologists must now adopt their new meaning. Is that not evidence that the cladists are on a crusade? To turn around and then accuse some Darwinians of being on a crusade because they do not accept the Hennigians’ imposition is indeed quite unjust.

Yes there is a Darwinian school of taxonomy, because Darwin is unambiguous about how organisms ought to be classified in a "natural system." Darwin has sometimes been misinterpreted as being a cladist in his classificatory philosophy. Nothing can be further from the truth. Darwin wrote:

“But I must explain my meaning more fully. I believe that the arrangement of the groups within each class, in due subordination and relation to each other, must be strictly genealogical in order to be natural; but that the amount of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by the forms being ranked under different genera, families, sections, or orders. the reader will best understand what is meant, if he will take the trouble to refer to the diagram in the fourth chapter.”

As one can see, Darwin is advocating the ranking of taxa based on “degrees of modification” or morphological disparity, which is of course a criterion that is rejected by the cladists, who only want to classify organisms according to branching order and nothing else. In fact, Darwin is advocating the recognition of paraphyletic taxa.

Darwin, referring to the only diagram in his book, continues:

“The forms descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the existing genus F14 may be supposed to have been but slightly modified; and it will then rank with the parent-genus F; just as some few still living organic beings belong to Silurian genera.”

By putting A and its descendants in different genera, Darwin has in effect created a paraphyletic genus which contains A but not all of its descendants. He did the same for I. Darwin therefore clearly shows that he has no philosophical aversion to paraphyletic taxa, unlike the Hennigians. In fact, he makes it quite clear that paraphyletic taxa are unavoidable if biologists are to classify organisms according to “degrees of modification” by giving us an exercise on how to create a Darwinian or "natural" classification.

johnscanlon:
“Suppose we would like to know “What species is/are the closest genealogical relatives of Green pythons?” or “What other species is/are most similar to Green pythons in morphology, ecology and behaviour?” What use is a classification going to be in finding answers? It depends: if we’re working with the ‘traditional’ classification that CKing prefers, with a distinct genus Chondropython, the answer is “none at all”, whereas if the species is called Morelia viridis and Morelia is a monophyletic group, we will be able to infer that Green pythons are most closely related (had most recent common ancestry with), and most likely quite similar in biology to (one, some or all) other
species of Morelia. In either case, it may be necessary to get behind the classification to the cladogram (or more complicated phylogeny) and raw data, but this is an easier process with one set of names than the other.”

Me:
If you want to answer the first question, it can be answered with a tree. Darwin drew trees. Darwinians draw trees. Cladists draw trees. Trees are useful in depicting relationships. Use them. If one wants to answer the second question, then one way that has proven useful is to classify it in a different higher taxon.

johnscanlon:
"Keeping the name ‘Chondropython’ is a bit like Steven King going back to writing under pseudonyms: the product is simply WORTH MORE, these days, if the name on the cover tells you where it actually comes from. Morelia viridis (or the M. viridis complex) is in fact a highly derived member of the M. spilota group, not a creation ex nihilo."

Me:
If Chondropython is so highly derived, then it should be removed from the same genus as its ancestor, as Darwin did by removing A’s descendants from the genus which includes A. By doing so, we are showing that Chondropython is quite distinct morphologically, behaviorally and ecologically than Morelia. Of course, removing highly derived members from a group is strictly prohibited by those who adhere religiously to Hennig's principle of holophyly. You asked, "When, if ever, are paraphyletic taxa useful?" The answer is that they are useful when one wishes to show that one or more members of a holophyletic taxon is highly derived.

Yes it is indeed possible to define taxa as the descendants of some ancient species, such as A in Darwin’s diagram. Such definitions, however, tell us nothing about the morphology, ecology and behavior of this group, which is what you said you are interested in. Therefore, an adherence to Hennigian dictates actually makes it less possible to answer some of the interesting biological questions that you posed.

Even if there is a good degree of certainty that a particular genealogy is correct, as is the case in Darwin’s hypothetical tree, he still advocates taxa be ranked on the basis of not just genealogical relationships but degrees of modification as well. Doing so of course meant the recognition of paraphyletic taxa. Not only are paraphyletic taxa the inevitable result of the process of evolution, as Carroll pointed out, but it is also the inevitable result of classifying organisms according to both genealogy and the degrees of modification, as Darwin demonstrated with his diagram.

Below is a very well drawn version of Darwin's only diagram in Origin:

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