WW wrote:

'What you are calling "ideological dissensions" may have more to do with issues concerning the interpretation of data and the need (or otherwise) for the change."



Me:

Precisely! Different schools of taxonomy will differ as to whether a particular set of data requires a change in the taxonomy. A Darwinian taxonomist, for example, has no problem recognizing paraphyletic taxa, such as Reptilia. A cladist, however, is intolerant of paraphyletic taxa. So taxonomists from these two different schools may treat the same data set quite differently.



WW:

'For instance, however good an mtDNA phylogeographic study is, it cannot truly elucidate the nature of contact zones without additional evidence, so many would be wary of proposals to split contiguous populations into species solely on the basis of an mtDNA phylogeny.'



Me:

I agree that mtDNA data cannot be used to delimit species. MtDNA is a more or less neutral character. Mutations in this molecule occurs randomly and at regular intervals. Hence it is a very good molecule to use as a molecular clock. If speciation were to occur at regular intervals like mutations in neutral molecules, then mtDNA would be very useful for predicting when speciation may have occurred in the past. Unfortunately, organismal evolution occurs independently of molecular evolution, a fact that even pioneer molecular systematists like Allan Wilson and his students and colleagues are fully aware of, hence it is not possible to equate mtDNA mutations with speciation events.



WW:

'Classification is not about differences, it's about reflecting phylogeny. If the Indo-Australian pythons are monophyletic, then lumping them into one genus is one way of dealing with that result. Of course, later work by Kluge arrived at a very different conclusion regarding the phylogeny of the pythons.'



Me:

That is of course the ideology of the cladists, who have decided to ignore morphological disparity when classifying organisms. Cladists only want to recognize holophyletic groups, which consist of one ancestor and all of the descendants of that ancestor. Holophyletic groups unfortunately tend to be heterogeneous, and they are not very useful for biologists who do not specialize in systematics. For example, reptiles mammals birds form a holophyletic group called the Amniota. However, this group has been split into three different taxa traditionally, namely Reptilia, Mammalia and Aves. Under the traditional arrangement, Reptilia is "paraphyletic" according to the cladists, since Reptilia do not consist of a single ancestor and all of the descendants of this ancestor. Some cladists have decided to eliminate paraphyly by including birds in their "Reptilia" while maintaining Mammalia as a separate group. Such a definition of "Reptilia" is of course more heterogeneous than the traditional usage, but it is the direct result of the kind of classificatory philosophy promulgated by WW and his cladistic colleagues. Both the Darwinians and the cladists have accused the others of recognizing groups that are not natural and are similar to groups that consist of "my car and three geese." Unless one is a cladist, it would be fair to say that it is clear which particular school is guilty of recognizing groups that consist of "my car and three geese." The cladistic definition of Reptilia (which includes birds) would appear to fit that description more than traditional Reptilia (which excludes birds). As a further example of the heterogeneity of cladistic taxa, simply recall Frost and Etheridge's "Chamaeleonidae", which combines the traditional Agamidae and Chameleonidae into a single family. Even more absurd is the proposal by some paleontologists that the whales and the hippopotamus be classified as a taxon called "Whippomorpha."



These examples do show that there exist major philosophical differences between different schools of systematists. There simply is no hope that these different schools will ever find a middle ground. Hence we are in for decades of taxonomic chaos to come.

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