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Hennig developed the cladistic method on the basis of the assumption that new species originate by the splitting of a stem species into two daughter species, hence on the principle of dichotomy: The parental species disappears with the birth of the two daughter species. He saw the origin of new higher taxa in an equivalent manner as a dichotomous process. Much research of the last 50 years has indicated, however, that budding is a far more frequent way of originating new taxa than is splitting. When a new species originates by means of peripatric speciation, this has no effect on the parental species from which the neospecies has budded off. However, by Hennig's criteria, a species which has given rise to a new species by budding thereby becomes paraphyletic and has to be removed from the classification, even though it has not been affected by the budding event. The same alignment pertains to higher taxa, most of which evidently originated by the budding off of an enterprising new species that was successful in a new niche or adaptive zone. The parental taxon continued to flourish unchanged in its traditional niche, but it has become paraphyletic by cladistic definition and must be excluded from the classification. It is now fully evident that the proposal to disqualify paraphyletic groups from recognition in classifications is not only impractical and destructive but scientifically untenable.--Ernst Mayr and Peter Ashlock 1991, Principles of Systematic Zoology, 2nd. Ed.

Below are two diagrams to illustrate Mayr and Ashlock's point. Both diagrams are based on mtDNA data from the same paper. As one can see, S. hammondii did not become extinct when S. bombifrons evolved. S. hammondii survived, unaltered by the speciation event giving rise to S. bombifrons, through peripatric speciation. Likewise, the family Pelobatidae did not become extinct when some member of the ancestral family gave rise to the family Pelodytidae. mtDNA data therefore shows that Hennig was wrong. Not only do species taxa not become extinct when new species evolve, higher taxa too follow the same pattern of evolution.

The Hennigians nevertheless simply ignore the fact that Hennig was wrong. The Hennigians (aka cladists) continue their crusade to eliminate paraphyly, even though Rober L. Carroll (1988 Vertebrate Paleontology and Evolution, p. 13) points out that paraphyletic groups are the inevitable result of the process of evolution. In the paper by Garcia-Paris et al., they sought to eliminate paraphyly by removing some members of the Pelobatidae and placing them in the family Scaphiopodidae. Such a proposal is both destructive and scientifically untenable, as Mayr and Ashlock pointed out. Such a proposal also creates two different families that are morphologically very similar to one another and their common ancestor, while obscuring the close relationship between the "scaphiopodids" and pelobatids. Their proposal also means that the common ancestor of the pelobatids, "scaphipodids" and pelodytids cannot be classified in any of these three families. A sane, taxonomically stable and scientifically tenable alternative is to continue to recognize the Pelobatidae as a paraphyletic taxon while maintaining the Pelodytidae as a valid taxon.

A great deal of taxonomic chaos being generated by the cladists' destructive and scientifically untenable intolerance of paraphyletic taxa can be avoided if they only realize that Hennig was simply wrong about evolution.

CKing: I am afraid your understanding of what Hennig said and what cladists practice are incorrect. I recommend reading Hennig's book Phylogenetic Systematics, go to part III, then go to the subheading "Dichotomy and Radiation". There you will find Hennig writing the opposite of what you think cladists believe about the pattern of evolution. The dichotomous cladograms are only an operational way to visualize sister relationships and are in no way an assumption or constraint about how things must evolve. This has been explicated numerous times in the literature.

Have a nice Thanksgiving (if you are reading this in the US). Cheers to the rest.

BIC

BIC wrote:
"The dichotomous cladograms are only an operational way to visualize sister relationships and are in no way an assumption or constraint about how things must evolve. This has been explicated numerous times in the literature."

Me:
Yes I understand that Hennig and his followers have admitted that his assumption of splitting is only a "convention." That is of course why Mayr said that the cladistic practice of disqualifying paraphyletic taxa is scientifically untenable. What you seem to be telling me is that even though the cladists know that paraphyletic groups are the inevitable result of the process of evolution, such knowledge has not persuaded the cladists to abandon their continuing crusade to eliminate paraphyletic taxa. I know that too.

Interesting answer. Although I am still not sure you have actually read Hennig. More importantly, I guess your reply means that you have taken back that statement that Hennig and cladists "misunderstand evolution." Excellent.

As for your view of the utility of paraphyly, I suggest that you submit your ideas to Systematic Biology for publication. That is not exaclty a cladist friendly journal so by doing so you can find out just how widespread your view of paraphyly is among systematists.

Cheers,

BIC

BIC made 2 points:

1. I have "taken back that statement that Hennig and cladists 'misunderstand evolution'".

2. My tolerance of paraphyly is not widely held among systematists.

I do agree with BIC that intolerance of paraphyletic taxa is fashionable and popular among many practicing systematists. Of course, that does not mean that such intolerance has any scientific basis. As I pointed out in my original post, Mayr and Ashlock have pointed out that the cladists' disqualification of paraphyletic taxa is impractical, destructive and scientifically untenable. BIC does not disagree and offers no rebuttal to Mayr and Ashlock's claim. So, if it is fashionable to be impractical, destructive and scientifically untenable, then the cladists will do it. What does that say about the cladists, or those systematists who adhere to cladistic ideology even if they are not cladists? I would say that they are like Galileo's proverbial starlings, who flock and great numbers but befoul the taxonomic literature with impractical, destructive and scientifically untenable taxnomic proposals.

As to whether or not Hennig and his followers understand evolution. It is difficult to ascertain. Their taxnomic practice suggests that they do not. Although if they are reasonably educated, they should know that budding evolution is the most frequent means by which new species evolve. They should also know that budding evolution results in paraphyletic groups and thus their intolerance of paraphyly is irrational. Hence there is a real conflict between scientific fact and cladistic ideology. The cladists have decided that they should side with ideology and ignore scientific fact. And judging by the number of systematists who are doing that, it is simply unsettling to know how many practicing systematists will ignore scientific fact. They are even so arrogant to suggest that they must be correct because so many people are practicing the same ideology.

If Hennig hadn't openly admitted that he ignored scientific facts, then would we have known that he actually knew what he has been ignoring? Is there a real difference between those who do not understand evolution and those who do understand evolution but ignores what they know about it?

After much contemplation, I would maintain that if the cladists act as though they do not understand evolution in their taxonomic practices, then they really do not understand evolution.

CKING wrote:

1. I have "taken back that statement that Hennig and cladists 'misunderstand evolution'".

2. My tolerance of paraphyly is not widely held among systematists.

Me: Excellent on both counts. That's progress. You may be surprised to know that Ashlock and Mayr textbook is not the end of discourse on the subject of paraphyly. Could it really be that the rest of the phylogenetics world has no grounds for considering paraphyly as a peril to classification? Is it truly possible that the rejection of paraphyly is only about fashion? Those are a lot of independent thinking scientists you disparage with that statement. Instead of the Galileo analogy you use, I suggest the analogy of the Flat Earth Society or believers in alien abduction.

Can I recommend an intereting book? David Hull wrote a book titled Science as a Process (1988 I think), in which he followed the revolution in systematics from the late 60's into the 80's. I think you would like it because you may find your justification for social issues dictating acceptance of new ideas in science. But you also may find that social issues aside, the bottom line for the acceptance of new ideas is the continued failure to reject the new ideas.

Cheers,
BIC

BIC wrote:
"Could it really be that the rest of the phylogenetics world has no grounds for considering paraphyly as a peril to classification? Is it truly possible that the rejection of paraphyly is only about fashion? Those are a lot of independent thinking scientists you disparage with that statement. Instead of the Galileo analogy you use, I suggest the analogy of the Flat Earth Society or believers in alien abduction."

Me:
If paraphyletic taxa are a "peril to classification," then I fail entirely to see what that "peril" may be. As R.L. Carroll pointed out, paraphyletic groups are the inevitable result of the process of evolution. Unless one is opposed to the very idea of evolution, paraphyletic taxa should never be a "peril" to biological classification, which must of course conform to the facts known to science as evolution. The rejection of paraphyletic taxa by Hennig is not of course "fashion." After all, he was alone when he formulated his principle of holophyly. The rest of the biological community had no problem with paraphyletic taxa, such as Reptilia. Since Darwin argued that birds evolved from a reptile, he of course knew that Reptilia does NOT include all of the descendants of the common ancestor of the reptiles. Darwin, therefore, knew that Reptilia was "paraphyletic" sensu Hennig, but he apparently never lost any sleep over it. The rest of the scientific community knew that Reptilia was "paraphyletic" ever since the discovery of Archaeopteryx, and yet no one was bothered by that fact until Hennig came along and declared paraphyletic taxa unacceptable.

So, you misunderstand. It is merely fashionable for many systematists to blindly follow Hennig's principle of holophyly by rejecting paraphyletic taxa, not that there is any scientific justification for disqualifying paraphyletic taxa.

Yes there are many independent thinkers among scientists, but the majority of the cladists are not among them. There are indeed a few independent thinkers among the cladists, and they tolerated paraphyletic taxa, but these cladists are viewed with suspicion by other, more orthodox cladists.

BIC:
But you also may find that social issues aside, the bottom line for the acceptance of new ideas is the continued failure to reject the new ideas.

Me:
If there is any continued failure, it is the cladists' failure to see that some of their ideas, such as their intolerance of paraphyletic taxa, are scientifically untenable. Further, it appears that some cladists are beginning to soften their irrational, hard line stance against paraphyletic taxa. For example, the family Agamidae, which was once disqualified because it was thought to be paraphyletic, is now being recognized by the very cladists who had lumped it into the same family as the Chamaeleonidae more than a decade ago.

HULL, DAVID L. (1988): SCIENCE AS A PROCESS an Evolutionary Account Of the Social and Conceptual Development of Science. THE UNIVERSITY OF CHICAGO PRESS, CHICAGO, ILL. [see also 1990 reprint]