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WW
at Tue Oct 26 06:55:01 2004 [ Report Abuse ] [ Email Message ] [ Show All Posts by WW ]
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>>Consensus is what develops over time after a proposal is first made in the literature - an increasing proportion of workers will either use or disregard the new arrangement. Also, where you look will make a difference. If a new taxonomic proposal is well supported, then academic publications will usually accept it quickly, whereas it takes a bit longer to percolate into the herpetocultural literature and when it comes to the taxonomic literature - consensus becomes synonymous with intertia. The average toxinological journal comes straight out of the 1960s when it comes to the taxonomy used.
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>>Ok, so far so good, but what makes them disregard a given proposal?
Who do you mean by "them"? If you mean toxinologists, it's because they are pig-ignorant and don't bother reading the literature.
If you mean other systematists, then there are avariety of reasons, usually to do with the data and/or interpretation thereof.
>>I understand that weak arguments or results provided in a proposal make other workers do so, but proposals with adequate data are also often discareded, because of more or less "ideological dissensions".
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. 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.
>>On the other hand evidence-free papers are widely accepted. I remember Stull (1935) placing the white-lipped python (L. albertisii Peters & Doria 1878) as subspecies of Liasis fuscus Peters 1873 without providing neither reasoning nor any evidence. This proposal was widely accepted (Loveridge 1948, de Haas 1950, Brongersma 1951, 1953 and 1956).
Sometimes simply because it was convenient at the time. Lumping was certainly in vogue in the middle 20th century, and the standards of evidence required then were not up to today's levels. If such a proposal were published today, then more evidenec would be required to make it "stick". The other question that has to be asked is what evidence existed at the time that albertisii was NOT conspecific with fuscus.
>> And then Underwood & Stimson (1990) in their phylogenetic study placed all the indo-australian python species recognized at that time to a single genus Morelia, ignoring the obvious morphological differences.
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.
>> So, what is "consensus" then? Is it the collective loose of one's way in taxonomy and forgetting the things learned once?
No. It is numbers of knowledgeable individuals agreeing on an issue on the basis of the evidence available. If there is little conclusive evidence, then you won't find consensus.
>>Your wrote:
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>>First, let's distinguish between species concept ("what is a species" and species diagnosis criteria ("how do we diagnose it?"
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>>Under just about any evolutionary paradigm, a species is basically a separate evolutionary lineage. Most of the so-called "species concepts" such as the BSC are in reality centered around diagnostic criteria - the BSC simply states that reproductive compatibility is the criterion for recognising species. I think pretty much everyone would agree that reproductive isolation DOES indicate separate species status, since two lineages that cannot exchange genes are clearly evolving separately. However, lack of reproductive isolation is no loger viewed as precluding separate species status by most practicing systematists nowadays, partly because of the impossibility of relevant testing for allopatric taxa.
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>>I do understand that it is impossible to test reproductive isolation of allopatric populations in the wild, and because of that, reproductive isolation is no longer being viewed as a creterion for speciation. Anyway, I tend to agree with Mayr (1996) that allopatric populations form incipient races and that "They may due in time aquire the needed isolating mechanisms to function as well separated species." (Mayr, 1996).
They are functioning as separated species because they do not exchange genetic material. Isolating mechanisms are irrelevant to any discussion of species limits in forms that do not meet in nature.
>>But by agreeing to Mayr's first statement, I also have to agree that "Owing to the gradualness of the process of speciation, every incipient species at one time in its cycle goes through subspecies stage." (Mayr, 1996). Am I wrong, or would it be so, that if one follows Mayr's statements, every allopatric population would at least be recognized at sub-generic level? It's only a matter of time until they will have to be recognized as full species?
Whether subspecies should be recognised or not is a different kettle of fish altogether. Clearly, where you decide that two allopatric populations are different species or not is a difficult question. You could argue that two amphibian ponds separated by a motorway are in effect in different evolutionary trajectories, since geentic exchange will be interrupted by the near 100% risk of dying from Dunlop disease. However, few people would argue that they should be recognised as separate species.
The fact is that after geographic separation, populations will as a default option drift apart genetically, morphologically, and reproductively, but in no particular order. As they drift apart, the various criteria used for species diagnosis under different "species concepts" will start to be fulfilled but again, not in any particular order. So, for instance, you may have ecological and/or morphological differences evolving very rapidly if the two daughter species occur in different environments, but reproductive compatibility may persist much longer, especially since there will be no selection for isolation due to reduced hybrid fitness in allopatric populations. On the other hand, if the daghter populations continue to exist in identical habitats, morphological and ecological change may be minimal, but genetic changes (e.g. coalescence of mtDNA haplotype lineages) may occur.
By far the best discussion of species conceptsand species diagnosis is:
De Queiroz, K. (1998) The general lineage concept of species, species criteria, and the process of speciation. Pp. 57-75. In Howard, D.J. & Berlocher, S.H. (Eds.), Endless Forms. Species and Speciation. Oxford University Press, New York.
Email me if you want a PDF.
Cheers,
Wolfgang
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