[Taxacom] Phylogenetic classification?

Kenneth Kinman kennethkinman at webtv.net
Fri Jul 31 21:45:41 CDT 2009

Hi Richard, 
       Such an expectation by some cladists that
ancestor-daughter species "inexorably" become sister species seems to be
largely wishful thinking. This might tend to occur in cases where the
daughter species has a population that is not greatly smaller than the
population of the mother species. However, such an expectation seems
rather unlikely in frequent speciation where there some element of
"founder effect", especially when the mother species is highly
polytypic. This would be most evident in extreme cases where the founder
is a single pregnant female (such as for metazoans) or even a single
seed (for plants) which manages to survive in a new environment and is
reproductively isolated.          
      The paraphyly is even more evident when one adds the time
dimension and considers the earlier history of the mother species (which
would usually be broader both genetically and phenotypically) than it
was at the later time when it gave rise to its daughter species.  If I
was a strict cladist, I would find it VERY disturbing to conceive of one
sister species having perhaps evolved a considerable period earlier than
its sister species.  Given the lack or sparsity of a fossil record
(along the stem between nodes) leading to most such species pairs,
having no good evidence of relative age makes it even more problematic.   
       Extinction and a poor fossil record is truly a double-edged sword
that can make the recognition of clades both useful in the face of that
lack of information, but often only temporarily useful and subject to
destablizing challenges to that cladistic recognition when fossil
intermediates are eventually discovered.  The trick is to occasionally
use paraphyly where cladistic assumptions seem likely to be overturned
by new information (fossil or otherwise).  Paraphyletic speciation (and
paraphyly at higher taxonomic levels) is too common for strict cladism
to continue hoping that it won't cause them any major problems (and
classificatory unstability for us all).  Stability and usefulness
require the occasional use of paraphyletic taxa at appropriate points in
the Tree of Life.
      ---------Ken Kinman
Richard Zander wrote: 
          As far as I can figure it out,
phylogeneticists expect a species that is paraphyletic (many exemplars
with a different species coming out of the middle of the lineage of
exemplars) to eventually become a sister group (reciprocally
monophyletic is the phrase). Therefore, a paraphyletic species should be
considered different from the autophyletic species because it will
inexorably become a sister group to it as exemplars get their act
together and homogenize their molecular data through recombination and
gene conversion and whatnot.

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