Polytomies

Warren Frank Lamboy wfl1 at NYSAES.CORNELL.EDU
Mon Jun 16 07:56:56 CDT 1997


To Taxacom:  In response to Eric Zurcher's questions, my commentary in
Systematic Botany 21: 243-253 (1996) discusses polytomies, their meaning,
interpretation, and the fact that "a polytomy in an evolutionary tree
simply represents an ancestral species giving rise to two or more
descendant species" (p. 247), and "a polytomy in a phylogenetic tree" means
that "it is not possible to resolve sister group relationships at the node
in question" (p. 247)  "Forcing a dichotomous resolution of a polytomy when
that dichotomy is an incorrect representation of reality does not seem to
me to be a particularly desirable goal."  (p. 247)  The rest of the paper
fleshes out these issues and more.

Best wishes-


Warren


At 09:20 AM 6/16/97 +1100, you wrote:
>At 08:51 AM 6/13/97 +0200, Thomas Pape <thomas.pape at NRM.SE> wrote:
>
>>Resolution-defying polytomies equal lack of knowledge and cannot be in
>>favour of tolerating paraphyly...
>
>Why is it that many cladists insist on regarding ALL polytomies as
>"resolvable", and that all we need is more knowledge to resolve them? I see
>no convincing reason why REAL polytomies cannot occur. (Indeed, given the
>"bursty" nature of evolutionary history seen in the fossil record, I
>suspect that real polytomies may in fact be rather common.)
>
>Here is a hypothetical (or perhaps not so hypothetical) scenario in which a
>polytomy would arise. Consider the Great Basin region of Utah and Nevada,
>with its series of dozens of parallel, but separate, mountain ranges.
>Imagine that during a period with a relatively cool, moist climate, a
>single species (belonging to whatever your favourite higher taxon may be)
>occurs over the entire region. The climate then shifts suddenly to become
>hotter and drier, and  only the mountain ranges now support suitable
>habitat for our species -- that is, what had been continuous habitat is
>broken up into a series of discrete islands. On several of the now-isolated
>range, our "species" undergoes some evolutionary change, enough so that
>many of the isolated populations might be regarded as new species. Any
>attempt to "resolve" the polytomy of this new set of species would only be
>misleading nonsense.
>
>Do any die-hard cladists wish to set me straight on this matter? (And after
>they do, perhaps we could discuss the problems which cladistics faces with
>hybridization.)
>
>Cheers,
>
>Eric Zurcher
>CSIRO Division of Entomology
>Canberra, Australia
>E-mail: ericz at ento.csiro.au
>
>
Senior Research Associate, Cornell University
USDA-ARS Plant Genetic Resources Unit
Geneva, New York  14456-0462
wfl1 at cornell.edu
VOX:    315-787-2359
FAX:    315-787-2339




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