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Re: Taxonomy is a matter of evidence...most of the time


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Posted by WW on October 18, 2002 at 04:37:12:

In Reply to: Re: Taxonomy is a matter of evidence...most of the time posted by Langaha on October 17, 2002 at 13:58:39:

::Burbrink et al.'s study showed extremely convincingly that the conventional subspecies are not real evolutionary entities - they have been blown out of the water, both by morphology and by mtDNA, end of discussion.

:Come on Wolfgang, I know you must know better than this. In many systematists minds, they haven't demonstrated anything more than population structure using a single slowly-evolving gene (cyt b)

Actually two genes - cytb and control region. Cytb is not all that slow anyway. In any case, the number of genes used is not all that important, since all mtDNA genes are inherited as a single linkage unit and thus have the same history. As far as reconstructing the phylogenetic history of the molecule is concerned, the data and statistical support they provide suggests that they have adequately represented this gene tree.

: to create a phylogeny. They need more genes and characters!

More mitochondrial genes will not make much of a difference - using AFLPs or microsats would indeed be useful to determine patterns of genetic exchange between the various taxa.

As to more characters - well, Burbrink's later study in Hepretological Monographs did use a large number of morphological characters and did demonstrate that the pattern of variation fits in better with the mtDNA phylogeny than with the conventional subspecies. I just happen to think that he did not demonstrate that the three main lineages are really full, separately evolving species, due to lack of attention to gene flow and intergrades.

: How can mtDNA be used to sink subspecies when gene flow will obscure their boundaries and prevent them from attaining reciprocal monophyly?

I am not quite sure what you are getting at here. Perhaps we should frst explore what you would use as the concept for a subspecies. Many people have different ideas as to what should or should not be recognised as a ssp., and whether we should recognise ssp. at all. Obviously, if you take a superficially phenetic 1950s approach and simply call anything that shows some degree of differentiation over geography a subspecies, then sure, mtDNA can never prove or disprove them. However, most systematists would also argue that you are not recognising evolutionary entities, and therefore, recognising the subspecies at all is questionable.
Burbrink et al did demonstrate convinvingly that the conventional subspecies of E. obsoleta do not constitute historical lineages, and they do demonstrate a history of past population subdivision in the Elaphe obsoleta complex.

: The lack of reciprocal monophyly should not invalidate subspecies recognition.

Again, this depends on your concept of what a subspecies is or should be. Lack of support frommorphology certainly does help to invalidate it, even if you are in favour of a purely phenetic subspecies concept.

: Elaphe obsoleta is quite arboreal and is found at high elevations. The Appalachian Mountains are not a barrier to gene flow and neither is the Apalachicola River.

Agreed.

: Their data clearly demonstrate this for the river, but they failed to sequence snakes from the Appalachian valleys (and mountains) and adjacent eastern side. How convenient!

Agreed - this is a major sampling gap, and one that is critical for the case of whetehr these should be considered separate species or not. If you look at the distribution of samples, in most points near the contact zones, the nearest samples from each clade are several hundred km apart (Mississippi valley, Appalachians). Where sampling is denser (Louisiana, NW Florida), reciprocal monophyly is not upheld, and barriers are shown to be "leaky".

: No one will deny that subspecies have been misused in the past (and so have species concepts), but the subspecies concept is simply not operational within the ESC definition of reciprocal monophyly. They used an inappropriate technique to answer something it cannot (unless perhaps there are true "lineages" that have been separated for millions of years).
: And the morphological study excluded most of the data that was said to be analyzed, he simply cropped his data to fit his answer. How can a type come from an "area of taxonomic uncertaintly?" What a farce!

The method of analysis of the morphological study is something I disagree with as well.

Cheers,

Wolfgang




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