WW wrote:

'A lot of what is blamed in "ideology" is simply due to the fact that our ways of thinking about evolution have changed, partly as a result of the diveristy of different tools that we have available. When things like the BSC were being formulated in the mid 20th century, reproductive isolation and morphology were pretty much the only sources of evidence that were available to researchers. Today, we have access to a much greater variety of tools, which give us access to different types of information, particularly phylogenetic information. I would argue that it would be very sad and worrying if these new tools and new insights had NOT affected the way we view the natural world and classify it.'



Me:

I would agree that the availability of molecular techniques have revolutionalized systematics. I disagree that these new techniques have caused biologists to change their world views. The two most popular alternatives to the Biological Species Concept, namely the Phylogenetic Species Concept and the Evolutionary Species Concept, were both formulated decades before molecular techniques became available. The PSC, for example, was promulgated by Willi Hennig, whose entire methodology was formulated without the benefit of molecular techniques. In fact, when Hennig's methodology first became popular in the 1970’s, all of Hennig's followers were morphologists. Ironically, it is the availability of molecular techniques, which threatened the careers of many systematists who relied exclusively on morphology, which drove them to embrace Hennig's cladistic methodology in the first place. Even more amusing is that the cladistic methodology the morphologists embraced totally ignores morphological disparity when classifying organisms. It shows how desparate molecular techniques had made the morphologists.



Perversely, if the Hennigians have their way, there will be no need for morphologists since morphological disparity will no longer be considered when classifying organisms, and since molecular techniques have largely dominated systematics in recent years. By embracing cladistics, the morphologists have in fact made their own discipline irrelevant! Little wonder that the only discipline in which morphologists still rule is paleontology, in which molecular techniques are rarely applicable, since DNA data are rarely available from fossils.



WW wrote:

‘Cladists wpuld argue that since there is only one true phylogeny, we should attempt to reflect that in classification. This has become the most widely accepted view in systematics, whether one likes it or not.”



Me:

It is true that there is only one true phylogeny, but it is equally true that, as George Gaylord Simpson pointed out, phylogeny cannot be observed, and phylogeny has to be inferred, often from data that are not directly phylogenetic in nature. Since different systematists would most likely infer phylogeny differently, there is virtually no hope of achieving a consensus when one must classify organisms strictly on the basis of phylogeny. Paraphrasing Simpson, there is no known way of determining all the dichotomies (or other fractionations) in any phylogeny or their relative dates among fossils, making their classification both theoretically and practically impossible.



To be useful, classification must be stable. The sole purpose of the existence of the ICZN and its long lists of rules, for example, is the promotion of taxnomic stability. G.G. Simpson has, as Darwin and many Darwinians have done, expressed the view that to be stable and useful, a classification must take into account both phylogeny and morphological disparity. G.G. Simpson wrote:



“...I will state briefly what I believe to be both the consensus and the most sensible basic approach to classification. Classification cannot be a complete and detailed expression of phylogeny, but it should be consistent with a reasonable estimate of phylogeny. ...In a consistent classification, an inference will be that all the members of a taxon had a common ancestry, although, if the ancestry was a single species (it need not be) or even a single genus or family, that ancestry is not necessarily formally made a member of the descendant taxon. It is also a definitely established factor in phylogeny that some lineages and resulting taxa diverge more widely and evolve more rapidly than others. The degree of difference between taxa in their evolved or derived characters is therefore phylogenetic and is a useful criterion of categorical level. Classifications are artifacts constructed primarily for their usefulness in biological thought and communication. It is therefore desirable to be conservative regarding them and to change a classification radically only when it has in some way come clearly into conflict with the simple criteria stated in this paragraph.”



Reference

Simpson, George Gaylord 1983. Fossils and the history of life. Scientific American Books. W. H. Freeman and Company. New York San Francisco

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