Polytomies

Kip Will kww4 at CORNELL.EDU
Tue Jun 17 08:31:04 CDT 1997


>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.)

Is this really the "normal" situation? Can't we recognize that this is a
"true" polytomy only because careful study of the taxa and the biogeographic
history suggests it does not fit the dichotomous pattern? Likewise,
reticulation from hybridization is only recognizable because it contrast
with a background of "normally" reproducing taxa.

How do you recognize what is a polytomy? If you have the set of taxa from
the example above, knowing nothing a priori of their history, what
methodological means is available to conclude that the correct relationship
is best represented by a polytomy?


well, back to the beetles, so many, so little time....
kip




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