Polytomies

Eric Zurcher ericz at ENTO.CSIRO.AU
Mon Jun 16 09:20:20 CDT 1997


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




More information about the Taxacom mailing list